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                       PASS THROUGH TRUST AGREEMENT _____

                                Dated _________

                                     among

                            UNION TANK CAR COMPANY,


                                [PROCOR LIMITED]


                                      and


                               _________________,
                            as Pass Through Trustee







                                   $________



                                 ______________
                           _______ Pass Through Trust
                           Pass Through Certificates,
                                 Series _______



   2


            TABLE OF CONTENTS TO PASS THROUGH TRUST AGREEMENT _____





Section                                                                          Page
- -------                                                                          ----

                                   ARTICLE I

                                  DEFINITIONS

                                                                            
SECTION 1.1.  Definitions ........................................................  3
SECTION 1.2.  Compliance Certificates and Opinions  .............................. 13
SECTION 1.3.  Form of Documents Delivered to Pass Through Trustee ................ 14
SECTION 1.4.  Acts of Certificateholders  ........................................ 14


                                   ARTICLE II

                    ACQUISITION OF EQUIPMENT NOTES AND ETCs;
                       ORIGINAL ISSUANCE OF CERTIFICATES


SECTION 2.1.  Issuance of Certificates; Acquisition of Equipment Notes and ETCs .. 16
SECTION 2.2.  Declaration of Trust; Acceptance By Pass Through Trustee ........... 19
SECTION 2.3.  Limitation of Powers ............................................... 19
SECTION 2.4.  Sale of Equipment Notes Under Certain Circumstances ................ 19


                                  ARTICLE III

                                THE CERTIFICATES


SECTION 3.1.  Form, Denomination and Execution of Certificates ................... 19
SECTION 3.2.  Authentication of Certificates  .................................... 20     
SECTION 3.3.  Temporary Certificates  ............................................ 20
SECTION 3.4.  Registration of Transfer and Exchange of Certificates  ............. 21
SECTION 3.5.  Mutilated, Destroyed, Lost or Stolen Certificates  ................. 22
SECTION 3.6.  Persons Deemed Owners  ............................................. 22
SECTION 3.7.  Cancellation  ...................................................... 22
SECTION 3.8.  Limitation of Liability for Payments ............................... 22
SECTION 3.9.  Book-Entry and Registered Certificates  ............................ 23



                                   ARTICLE IV

                          DISTRIBUTIONS; STATEMENTS TO
                               CERTIFICATEHOLDERS
                                      

                                      -i-


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Section                                                                                         Page
- -------                                                                                         ----
                                                                                           
SECTION 4.1.  Certificate Account and Special Payments Account  ...............................  25
SECTION 4.2.  Distribution from Certificate Account and Special Payments Account ..............  26
SECTION 4.3.  Statements to Certificateholders ................................................  27
SECTION 4.4.  Investment of Special Payment Money  ............................................  28


                                   ARTICLE V

                                 THE COMPANIES


SECTION 5.1.  Maintenance of Corporate Existence  .............................................  28
SECTION 5.2.  Consolidation, Merger or Sale of Assets Permitted  ..............................  29


                                   ARTICLE VI

                                    DEFAULT


SECTION 6.1.  Events of Default  ..............................................................  30
SECTION 6.2.  Incidents of Sale of Equipment Notes and ETCs  ..................................  32
SECTION 6.3.  Judicial Proceedings Instituted by Pass Through Trustee .........................  32
SECTION 6.4.  Control by Certificateholders  ..................................................  33
SECTION 6.5.  Waiver of Past Defaults .........................................................  34
SECTION 6.6.  Undertaking to Pay Court Costs  .................................................  34
SECTION 6.7.  Right of Certificateholders to Receive Payments Not to Be
                 Impaired .....................................................................  34
SECTION 6.8.  Certificateholders May Not Bring Suit Except Under
                 Certain Conditions  ..........................................................  35
SECTION 6.9.  Remedies Cumulative .............................................................  35


                                  ARTICLE VII

                            THE PASS THROUGH TRUSTEE


SECTION 7.1.   Certain Duties and Responsibilities ............................................  36
SECTION 7.2.   Notice of Defaults  ............................................................  37
SECTION 7.3.   Certain Rights of Trustee  .....................................................  37
SECTION 7.4.   Not Responsible for Recitals or Issuance of Certificates .......................  38
SECTION 7.5.   May Hold Certificates  .........................................................  39
SECTION 7.6.   Money Held in Pass Through Trust  ..............................................  39
SECTION 7.7.   Compensation and Reimbursement  ................................................  39
SECTION 7.8.   Corporate Trustee Required; Eligibility  .......................................  40
SECTION 7.9.   Resignation and Removal; Appointment of Successor ..............................  41



                                      -ii-
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Section                                                                                Page
- -------                                                                                ----
                                                                                  
SECTION 7.10.  Acceptance of Appointment by Successor...............................    43

SECTION 7.11.  Merger, Conversion, Consolidation or Succession to Business..........    43
SECTION 7.12.  Maintenance of Agencies..............................................    43
SECTION 7.13.  Money for Certificate Payments to Be Held in Pass Through Trust......    45
SECTION 7.14.  Registration of Equipment Notes and ETCs in Pass Through
                 Trustee's  Name....................................................    45
SECTION 7.15.  Representations and Warranties of Pass Through Trustee...............    46
SECTION 7.16.  Withholding Taxes; Information Reporting.............................    47
SECTION 7.17.  Trustee's Liens......................................................    47
SECTION 7.18.  Preferential Collection of Claims....................................    47

                                  ARTICLE VIII

         CERTIFICATEHOLDERS' LISTS AND REPORTS BY PASS THROUGH TRUSTEE


SECTION 8.1.  The Companies to Furnish Pass Through Trustee with Names
                 and Addresses of Certificateholders................................    48
SECTION 8.2.  Preservation of Information; Communications to Certificateholders.....    48
SECTION 8.3.  Reports by Pass Through Trustee.......................................    48
SECTION 8.4.  Reports by the Company................................................    48


                                   ARTICLE IX

                  SUPPLEMENTS TO PASS THROUGH TRUST AGREEMENT

 SECTION 9.1. Supplements to Pass Through Trust Agreement Without
                 Consent of Certificateholders......................................    49
 SECTION 9.2. Supplements to Pass Through Trust Agreement with
                 Consent of Certificateholders......................................    50

SECTION 9.3.  Documents Affecting Immunity or Indemnity.............................    51
SECTION 9.4.  Execution of Supplements to Pass Through Trust Agreements.............    51
SECTION 9.5.  Effect of Supplements to Pass Through Trust Agreement.................    51
SECTION 9.6.  Conformity with Trust Indenture Act...................................    52

SECTION 9.7.  Reference in Certificates to Supplements to Pass
              Through Trust Agreements..............................................    52


                                   ARTICLE X

                    AMENDMENTS TO INDENTURE, NOTE DOCUMENTS,
                  EQUIPMENT TRUST AGREEMENTS AND ETC DOCUMENTS

                                    -iii-

                                                                
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Section                                                                 Page
- -------                                                                 ----

                                   ARTICLE XI
                       TERMINATION OF PASS THROUGH TRUST


                                  ARTICLE XII

                               GUARANTEE OF UNION


                                                                          
SECTION 12.1.           Guarantee...........................................     54
SECTION 12.2.           Execution and Delivery of Guarantee.................     55
SECTION 12.3.           Limitation of Union's Liability.....................     55
SECTION 12.4.           Guarantee Unconditional.............................     55


                                           ARTICLE XIII

                                     MISCELLANEOUS PROVISIONS


SECTION 13.1.           Limitation on Rights of Certificateholders..........     57
SECTION 13.2.           Certificates Nonassessable and Fully Paid...........     57
SECTION 13.3.           Notices.............................................     57

SECTION 13.4.          Communication by Certificateholder with Other
                         Certificateholders.................................     59


SECTION 13.5.           Governing Law.......................................     59
SECTION 13.6.           Severability of Provisions..........................     59
SECTION 13.7.           Trust Indenture Act Controls........................     59
SECTION 13.8.           Effect of Headings and Table of Contents............     59
SECTION 13.9.           Successors and Assigns..............................     59
SECTION 13.10.          Benefits of Pass Through Trust Agreement............     60
SECTION 13.11.          Legal Holidays......................................     60
SECTION 13.12.          Counterparts........................................     60

Exhibit A     -  Form of Certificate
Exhibit B     -  Guarantee
Exhibit C     -  Form of DTC Letter of Representations
Schedule I    -  Description of Equipment Notes to be Issued under the Indenture
Schedule II   -  Description of ETCs to be Issued under Equipment Trust Agreements

                                        

                                        
                                     -iv-

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            This PASS THROUGH TRUST AGREEMENT _______, dated __________,
       is made with respect to the formation of the Union Tank Car
       Company ______ Pass Through Trust, among UNION TANK CAR COMPANY,
       a Delaware corporation ("Union"), [PROCOR LIMITED, a Canadian
       corporation and an indirect wholly-owned subsidiary of Union
       ("Procor" and together with Union, the "Companies")] and
       __________, a national banking association solely as Pass Through
       Trustee and not in its individual capacity.


                                  WITNESSETH:


            [WHEREAS, a $_______ aggregate principal amount Equipment
       Trust Certificate, Series ___ (together with the guarantee of
       Union to be endorsed thereon, the "Union ETCs"), will be issued
       pursuant to an Equipment Trust Agreement, dated ______ as may be
       amended or supplemented from time to time (the "Union Equipment
       Trust Agreement"), between Union and ___________, as trustee (the
       "Union Equipment Trust Trustee");]

            [WHEREAS, a $________ aggregate principal amount Equipment
       Trust Certificate, Series _____ (together with the guarantee of
       Procor endorsed thereon, the "Procor ETC" and, together with the
       Union ETCs, the "ETCs"), will be issued pursuant to an Equipment
       Trust Agreement, dated ________, as may be amended or
       supplemented from time to time (the "Procor Equipment Trust
       Agreement") between Procor and ___________, as trustee (the
       "Procor Equipment Trust Trustee");]

            [WHEREAS, the Union Equipment Trust Trustee will issue the
       Union ETCs in connection with the financing of railway tank cars
       and other rail cars of the type used in Union's business (the
       "Union Trust Equipment") and the Procor Equipment Trust Trustee
       will issue the Procor ETC in connection with the financing of
       railway tank cars and other rail cars of the type used in
       Procor's business (the  "Procor Trust Equipment");]

            [WHEREAS, pursuant to the terms of this Pass Through Trust
       Agreement, the Union Equipment Trust Agreement and the Procor
       Equipment Trust Agreement, the Union ETCs and the Procor ETC are
       to be sold upon their issuance to the Pass Through Trustee, and
       the Pass Through Trustee shall purchase the ETCs upon their
       issuance and shall hold such ETCs in trust for the benefit of the
       Certificateholders;]

            WHEREAS, a certain Owner Trustee, on behalf of a certain
       Owner Participant, will purchase certain Equipment (as defined)
       from [Union] [Procor];

            WHEREAS, the Owner Trustee will lease the Equipment to
       [Union] [Procor] pursuant to the Lease;


 

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            WHEREAS, such Owner Trustee will issue on a nonrecourse
       basis Equipment Notes, under the Indenture, in order to finance
       not more than 80% of the purchase price to be paid to [Union]
       [Procor] for the Equipment subject to the Lease;

            WHEREAS, pursuant to the terms and conditions of this Pass
       Through Trust Agreement and the Participation Agreement, such
       agreements to be entered into by the Pass Through Trustee
       contemporaneously with the execution and delivery of this Pass
       Through Trust Agreement, certain Equipment Notes are to be sold,
       from time to time, to the Pass Through Trustee, and the Pass
       Through Trustee shall purchase, from time to time, such Equipment
       Notes and shall hold such Equipment Notes in trust for the
       benefit of the Certificateholders;

            WHEREAS, the Pass Through Trustee, upon execution and
       delivery of this Pass Through Trust Agreement, hereby declares
       the creation of this Pass Through Trust for the benefit of the
       Certificateholders, and the initial Certificateholders as the
       grantors of the Pass Through Trust, by their respective
       acceptances of the Certificates, join in the creation of this
       Pass Through Trust with the Pass Through Trustee;

            WHEREAS, to facilitate the sale of the [ETCs] [and] [certain
       Equipment Notes] to the Pass Through Trustee and the purchase of
       such [ETCs] [and] [Equipment Notes] by the Pass Through Trustee,
       [each of] [Union] [and] [Procor] has duly authorized the
       execution and delivery of this Pass Through Trust Agreement as an
       "issuer", as such term is defined in and solely for purposes of
       the Securities Act of 1933, as amended, of the Certificates being
       issued hereunder and as an "obligor", as such term is defined in
       and solely for purposes of the Trust Indenture Act of 1939, as
       amended, with respect to the Certificates and is undertaking to
       perform certain administrative and ministerial duties hereunder
       and is also undertaking to pay the fees and expenses of the Pass
       Through Trustee; and

            WHEREAS, this Pass Through Trust Agreement is subject to the
       provisions of the Trust Indenture Act of 1939, as amended, and
       shall, to the extent applicable, be governed by such provisions;

            NOW, THEREFORE, in consideration of the mutual agreements
       herein contained, and of other good and valuable consideration
       the receipt and adequacy of which are hereby acknowledged, the
       parties hereto agree as follows:


                                     -2-

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                                   ARTICLE I

                                  DEFINITIONS

            SECTION 1.1.  Definitions.

            (a)  For all purposes of this Pass Through Trust Agreement,
       except as otherwise expressly provided or unless the context
       otherwise requires:
                 (i)  the terms used herein that are defined in this
            Article have the meanings assigned to them in this Article,
            and include the plural as well as the singular;

                 (ii)  all other terms used herein which are defined in
            the Trust Indenture Act, either directly or by reference
            therein, have the meanings assigned to them therein;

                 (iii) all references in this Pass Through Trust
            Agreement to designated "Articles", "Sections" and other
            subdivisions are to the designated Articles, Sections and
            other subdivisions of this Pass Through Trust Agreement; and

                 (iv) the words "herein", "hereof " and "hereunder" and
            other words of similar import refer to this Pass Through
            Trust Agreement as a whole and not to any particular
            Article, Section or other subdivision.

            (b)  For all purposes of this Pass Through Trust Agreement,
       the following capitalized terms have the following respective
       meanings:

            "Act," with respect to any Certificateholder, has the
       meaning specified in Section 1.4.

            "Affiliate" of any Person shall mean any other Person which
       directly or indirectly controls, or is controlled by, or is under
       a common control with, such Person.  The term "control", as used
       with respect to any Person, means the possession, directly or
       indirectly, of the power to direct or cause the direction of the
       management policies of such Person, whether through the ownership
       of voting securities, by contract or otherwise; and the terms
       "controlling" and "controlled" have meanings correlative to the
       foregoing.

            "Authorized Agent" means any Paying Agent or Registrar.

            "Avoidable Tax" has the meaning specified in Section 7.9(e).

            "Book-Entry Certificates" means a beneficial interest in the
       Certificates, ownership and transfers of which shall be made
       through book entries by a Clearing Agency as described in Section
       3.9.


                                     -3-

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            "Business Day" means any day other than a Saturday or a
       Sunday or a day on which commercial banking institutions or trust
       companies are authorized or required by law, regulation or
       executive order to be closed in New York, New York, Chicago,
       Illinois [, or, solely with respect to payments under the Procor
       Equipment Trust Agreement, Toronto, Ontario, Canada], or, so long
       as any Certificate is outstanding, a city and state in which the
       respective Corporate Trust Office of the Owner Trustee, the Pass
       Through Trustee, the Indenture Trustee, any Equipment Trust
       Trustee or Paying Agent is located.

            "Certificate" means any one of the certificates executed and
       authenticated by the Pass Through Trustee, substantially in the
       form of Exhibit A hereto.

            "Certificate Account" means the account or accounts created
       and maintained pursuant to Section 4.1(a).

            "Certificateholder" means the Person in whose name a
       Certificate is registered in the Register.

            "Certificate Owner" means, when used in Section 3.9, the
       Person who owns a Book Entry Certificate.

            "Clearing Agency" means an organization registered as a
       "clearing agency" pursuant to Section 17A of the Securities
       Exchange Act of 1934, as amended.

            "Clearing Agency Participant" means a broker, dealer, bank,
       other financial institution or other Person for whom from time to
       time a Clearing Agency effects, directly or indirectly,
       book-entry transfers and pledges of securities deposited with the
       Clearing Agency.

            "Closing Date" means _______.

            "Commission" means the Securities and Exchange Commission,
       as from time to time constituted, created under the Securities
       Exchange Act of 1934, or, if at any time after the execution of
       this instrument such Commission is not existing and performing
       the duties now assigned to it under the Trust Indenture Act, then
       the body performing such duties on such date.

            "Companies" means Union Tank Car Company, a Delaware
       corporation, [and Procor Limited, a Canadian corporation,] or
       [its][their respective] successors in interest pursuant to
       Section 5.2, or any other obligor with respect to the
       Certificates (within the meaning of the Trust Indenture Act).

            "Corporate Trust Office" means, with respect to the Pass
       Through Trustee, any Equipment Trustee and the Indenture Trustee,
       the office of such trustee in the city at which at any particular
       time its corporate trust business shall be principally
       administered.


 
                                     -4-
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            "De Minimis Certificate" has the meaning specified in
       Section 3.1.

            "Direction" has the meaning specified in Section 1.4(c).

            "Equipment" means, with respect to the Union ETCs, all of
       the Union Trust Equipment, with respect to the Procor ETC, all of
       the Procor Trust Equipment, and with respect to the Equipment
       Notes, all of the Units covered by the Lease; or as the context
       may require, all of the Equipment covered by the ETCs and the
       Equipment Notes.

            "Equipment Note" means any one of the Equipment Notes (as
       defined in the Indenture) described on Schedule I attached
       hereto, including any Equipment Note (as so defined) issued under
       the Indenture as a replacement or substitution therefor, held by
       the Pass Through Trustee.

            "Equipment Trust Agreement" means [either of] the Union
       Equipment Trust Agreement [or the Procor Equipment Trust
       Agreement, each] dated ________, as [each] such Equipment Trust
       Agreement may be amended or supplemented from time to time in
       accordance with its [respective] terms[; and "Equipment Trust
       Agreements" means both of such agreements].

            "Equipment Trust Default" means, with respect to either
       Equipment Trust Agreement, any Event of Default (as such term is
       defined in such Equipment Trust Agreement).

            "Equipment Trust Trustee" means the Union Equipment Trust
       Trustee [and the Procor Equipment Trust Trustee].

            "ETC" means any one of the Equipment Trust Certificates
       described in Schedule II attached hereto, including any ETC (as
       so defined) issued under the applicable Equipment Trust Agreement
       as a replacement or substitution therefor, held by the Pass
       Through Trustee.

            "ETC Documents", with respect to an ETC, means the
       applicable Equipment Trust Agreement.

            "Guarantee" means the guarantee of Union set forth in
       Article XII hereof and on each of the Certificates.

            "Escrow Account" has the meaning specified in Section
       2.1(b).

            "Escrowed Funds" has the meaning specified in Section
       2.1(b).

            "Event of Default" means an event described in Section 6.1.

                                     -5-
 

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            "Fractional Undivided Interest" means the undivided interest
       in the Pass Through Trust that is evidenced by an Outstanding
       Certificate expressed as a fraction of the total undivided
       interests in the Pass Through Trust represented by all
       Outstanding Certificates.

            "Indenture" means the Trust Indenture and Security Agreement
       (UTC Trust No. _____) (___), dated _______, between the Owner
       Trustee and the Indenture Trustee, as the Indenture may be
       amended or supplemented from time to time in accordance with its
       terms.  The term "Indenture" includes each Indenture Supplement
       entered into pursuant to the terms of the Indenture.

            "Indenture Default" means, with respect to the Indenture,
       any Indenture Event of Default (as such term is defined in the
       Indenture).

            "Indenture Trustee" means ______________, in its capacity as
       indenture trustee under the Indenture, and any other Person which
       may from time to time be acting as Indenture Trustee in
       accordance with the provisions of the Indenture.

            "Initial Cut-off Date" means _________.

            "Issuance Date" means the date of the issuance of the
       Certificates.

            "Lease" means the Equipment Lease Agreement (UTC Trust No.
       ______) (____) with respect to the Equipment between the Owner
       Trustee, as the lessor, and Union, as the lessee, as the Lease
       may be amended or supplemented from time to time in accordance
       with its terms.  Such term shall include each Lease Supplement
       entered into pursuant to the terms of the Lease.

            "Lease Event of Default" means, with respect to the Lease,
       an Event of Default under the Lease as specified in Section 14
       thereof.

            "Lease Supplement" has the meaning assigned to that term in
       the Lease.

            "Letter of Representations" means the agreement among the
       Companies, the Pass Through Trustee and the initial Clearing
       Agency substantially in the form attached hereto as Exhibit C.

            "Make-Whole Amount" has the meaning assigned to that term in
       the Indenture.

            "Note Documents," with respect to any Equipment Note, means
       the Indenture, the Lease and the Participation Agreement.

            "Officer's Certificate" means a certificate signed, (a) in
       the case of either of the Companies, by (i) the Treasurer, any
       Assistant Treasurer, the Controller or any Assistant Controller
       of such Company, signing alone, or (ii) any Vice President
       signing together with the


                                     -6-

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       Secretary, any Assistant Secretary, the Treasurer or any
       Assistant Treasurer of such Company, or (b) in the case of the
       Owner Trustee, an Equipment Trust Trustee or the Indenture
       Trustee, a Responsible Officer of such Owner Trustee, Equipment
       Trust Trustee or Indenture Trustee.

            "Opinion of Counsel" means a written opinion of legal
       counsel, who (i) in the case of counsel for either of the
       Companies may be (A) a lawyer employed by such Company, (B) Neal,
       Gerber & Eisenberg, or (C) such other counsel designated by such
       Company and reasonably acceptable to the Pass Through Trustee and
       (ii) in the case of the Owner Trustee, any Equipment Trust
       Trustee or the Indenture Trustee, such counsel as may be
       designated by any of them whether or not such counsel is an
       employee of any of them, and who shall be acceptable to the Pass
       Through Trustee.

            "Outstanding," with respect to Certificates, means, as of
       the date of determination, all Certificates theretofore
       authenticated and delivered under this Pass Through Trust
       Agreement, except:

                 (i)  Certificates theretofore canceled by the Registrar
            or delivered to the Pass Through Trustee or the Registrar
            for cancellation;

                 (ii)  Certificates for which money in the full amount
            required to make the final distribution payment to be made
            pursuant to Section 11.1 hereof has been theretofore
            deposited with the Pass Through Trustee in trust for the
            Certificateholders as provided in Section 4.1 pending
            distribution of such money to the Certificateholders
            pursuant to such final distribution payment; and

                 (iii)  Certificates in exchange for or in lieu of which
            other Certificates have been authenticated and delivered
            pursuant to this Pass Through Trust Agreement.

            "Owner Participant" means the "Owner Participant" referred
       to in the Participation Agreement and any permitted successor or
       assign of any such Owner Participant.

            "Owner Trustee," with respect to any Equipment Note or the
       Indenture or the Lease, means _______________, not in its
       individual capacity but solely as owner trustee of an owner trust
       for the benefit of the Owner Participant, and each other Person
       which may from time to time be acting as Owner Trustee in
       accordance with the provisions of the Note Documents.

            "Participation Agreement" means the Participation Agreement
       (UTC Trust No. ______) (____), dated _________, and to which the
       Pass Through Trustee, the Owner Trustee, the Indenture Trustee,
       the Owner Participant, and Union are parties, as the
       Participation Agreement may be amended or supplemented from time
       to time in accordance with its respective terms.


                                     -7-

   13



            "Pass Through Trust" means the trust created by this Pass
       Through Trust Agreement, the estate of which consists of the
       Trust Property.

            "Pass Through Trust Agreement" means this Pass Through Trust
       Agreement, as it may be amended from time to time in accordance
       with the terms hereof.

            "Pass Through Trustee" means the institution executing this
       Pass Through Trust Agreement as Pass Through Trustee, or its
       successor in interest, and any successor trustee appointed as
       provided herein.

            "Paying Agent" means the paying agent maintained and
       appointed pursuant to Section 7.12.

            "Permitted Investments" means each of (i) direct obligations
       of the United States of America and agencies thereof; (ii)
       obligations fully guaranteed by the United States of America;
       (iii) certificates of deposit issued by, or bankers' acceptances
       of, or time deposits with, any bank, trust company or national
       banking association incorporated or doing business under the laws
       of the United States of America or one of the states thereof
       having combined capital and surplus and retained earnings of at
       least $100,000,000, having general obligations rated at least A1
       by Moody's Investors Service, Inc. or A+ by Standard & Poor's
       Corporation (but excluding any new investment as to which there
       is a public announcement by the rating agency providing a rating
       thereon that such rating is under consideration for a possible
       downgrade below A1 or A+, as the case may be), including the
       Owner Trustee in its individual capacity or the Indenture Trustee
       in its individual capacity if such conditions are met; (iv)
       commercial paper of any holding company of a bank, trust company
       or national banking association described in clause (iii); (v)
       bearer note deposits with, or certificates of deposit issued by,
       or promissory notes of, any subsidiary incorporated under the
       laws of Canada (or any province thereof) of any bank, trust
       company or national banking association described in clause
       (iii); (vi) commercial paper of companies having a rating of
       A-l/P-l or better assigned to such commercial paper by Standard &
       Poor's Corporation or Moody's Investors Service, Inc.  (or, if
       neither such organization shall rate such commercial paper at any
       time, by any nationally recognized rating organization in the
       United States of America); (vii) U.S. dollar-denominated
       certificates of deposit issued by, or time deposits with, the
       European subsidiaries of any bank, trust company or national
       banking association described in clause (iii); (viii) Canadian
       Treasury Bills fully hedged to U.S. dollars; (ix) bonds, notes or
       other obligations of any state of the United States of America,
       or any political subdivision of any such state, or any agencies
       or other instrumentalities of any such state, including, but not
       limited to, industrial development bonds, pollution control
       revenue bonds, public power bonds, housing bonds, other revenue
       bonds or any general obligation bonds; provided that, at the time
       of their purchase, such obligations are rated in the highest
       rating category by Standard & Poor's Corporation or Moody's
       Investors Service, Inc.  (or, if neither such organization shall
       rate such obligations at such time, by any nationally recognized
       rating organization in the United States of America); or (x)
       bonds or other debt instruments of any company, if such bonds or
       other debt


                                     -8-

   14


       instruments, at the time of their purchase, are rated in the
       highest rating category by Standard & Poor's Corporation or
       Moody's Investors Service, Inc. (or, if neither such organization
       shall rate such obligations at such time, by any nationally
       recognized rating organization in the United States of America);
       provided that no investment shall be eligible as and included
       within the definition of the term "Permitted Investment" unless
       either (x) the final maturity or date of return of such
       investment is equal to one year or less from the date of purchase
       thereof, or (y) in the case of any investment referred to in the
       foregoing clause (i) or (ii) only, such investment has a final
       maturity or date of return greater than one year from the date of
       purchase thereof and closing prices on a national securities
       exchange or bid and asked prices, closing prices or yields to
       maturity for such investment are reported in The Wall Street
       Journal (or if The Wall Street Journal is not at the time
       published or ceases to report such prices, such prices are
       reported by any other publication of nationally recognized
       standing of general circulation in New York City).

            "Person" means any individual, corporation, partnership,
       joint venture, association, joint stock company, trust,
       unincorporated organization, or government or any agency or
       political subdivision thereof.

            "Pool Balance" means, as of any date, the aggregate unpaid
       principal amount of the ETCs and the Equipment Notes held in the
       Pass Through Trust on such date plus the amount of the principal
       payments on the ETCs and the Equipment Notes held by the Pass
       Through Trustee and not yet distributed plus the amount of any
       moneys held in the Escrow Account (other than earnings thereon).
       The Pool Balance as of any Regular Distribution Date or Special
       Distribution Date shall be computed after giving effect to the
       payment of principal, if any, on the Equipment Notes held in the
       Pass Through Trust and distribution thereof to be made on that
       date.

            "Pool Factor" means, as of any date, the quotient (rounded
       to the seventh decimal place) computed by dividing (i) the Pool
       Balance by (ii) the aggregate original principal amount of the
       Certificates issued under the Pass Through Trust.  The Pool
       Factor as of any Regular Distribution Date or Special
       Distribution Date shall be computed after giving effect to the
       payment of principal, if any, on the ETCs and the Equipment Notes
       held in the Pass Through Trust and distribution thereof to be
       made on that date.

            "Postponed Notes" means the Equipment Notes or ETCs as to
       which a Postponement Notice shall have been delivered pursuant to
       Section 2.1(b).

            "Postponement Notice" means a certificate of Union signed by
       an officer of Union (i) requesting that the Pass Through Trustee
       execute and deliver the Participation Agreement but temporarily
       postpone payment of the purchase price of the Equipment Notes or
       ETCs to a date later than the Issuance Date, (ii) identifying the
       amount of the purchase price of each Equipment Note or ETC and
       the aggregate purchase price of all such Equipment Notes or ETCs,
       (iii) setting forth the reasons for such postponement and (iv)
       with respect to the Participation Agreement referred to in clause
       (i), either (A) setting or resetting a new Closing Date (which
       shall be on or


                                     -9-

   15


       prior to the Initial Cut-off Date) for payment by the Pass
       Through Trustee of such purchase price and issuance of the
       Equipment Note or ETC, or (B) indicating that such new Closing
       Date (which shall be on or prior to the Initial Cut-Off Date)
       will be set by subsequent written notice not less than one
       Business Day prior to such new Closing Date.

            ["Procor Equipment Trust Trustee" means __________, in its
       capacity as the trustee under the Procor Equipment Trust
       Agreement, and any other Person which from time to time will be
       acting as Equipment Trust Trustee in accordance with the
       provisions of the Procor Equipment Trust Agreement.]

            "Record Date" means the Business Day preceding a Regular
       Distribution Date or a Special Distribution Date, as applicable.

            "Register" has the meaning specified in Section 3.4.

            "Registered Certificates" has the meaning specified in
       Section 3.1.

            "Registrar" has the meaning specified in Section 3.4.

            "Regular Distribution Date" means, with respect to
       distributions of Scheduled Payments, _______ and _______ of each
       year, commencing on __________, until payment of all the
       Scheduled Payments to be made under the Equipment Notes and the
       ETCs has been made.

            "Request" means a request by Union, on behalf of the
       Companies, setting forth the subject matter of the request
       accompanied by an Officer's Certificate and an Opinion of Counsel
       as provided in Section 1.2 of this Pass Through Trust Agreement.

            "Responsible Officer" means (i) with respect to the initial
       Pass Through Trustee, any initial Equipment Trust Trustee or the
       initial Indenture Trustee, any officer in the Corporate Trust
       Office or any other office at which the Pass Through Trustee, any
       Equipment Trust Trustee or the Indenture Trustee conducts
       corporate trust business; and (ii) with respect to any successor
       Pass Through Trustee, successor Equipment Trust Trustee,
       successor Indenture Trustee or the Owner Trustee, the chairman or
       vice-chairman of the board of directors or trustees, the chairman
       or vice-chairman of the executive or standing committee of the
       board of directors or trustees, the president, the chairman of
       the committee on trust matters, any vice-president, any second
       vice-president, the secretary, any assistant secretary, the
       treasurer, any assistant treasurer, the cashier, any assistant
       cashier, any trust officer or assistant trust officer, the
       comptroller and any assistant comptroller.  Responsible Officer
       also means, with respect to the Pass Through Trustee, any
       Equipment Trust Trustee, the Indenture Trustee and the Owner
       Trustee, any other officer of the Pass Through Trustee, the Owner
       Trustee, such Equipment Trust Trustee or the Indenture Trustee
       customarily performing functions similar to those performed by
       the persons who at the time shall be any of the above designated
       officers, and with respect to a particular corporate trust
       matter,


                                     -10-

   16


       any other officer to whom such matter is referred because of his
       knowledge of and familiarity with the particular subject.

            "Scheduled Payment" means, with respect to a Regular
       Distribution Date, any payment (other than a Special Payment) of
       interest on or principal of and interest on:  (i) an Equipment
       Note due from the Owner Trustee which issued such Equipment Note,
       which payment represents the installment of principal at the
       stated maturity of such installment of principal on such
       Equipment Note, or the payment of regularly scheduled interest
       accrued on the unpaid principal amount of such Equipment Note, or
       both, to be made on such Regular Distribution Date pursuant to
       the terms of such Equipment Note; or (ii) an ETC, to be made on
       such Regular Distribution Date pursuant to the terms of such ETC.

            "Special Distribution Date" means (i) in the case of
       prepayments with respect to a voluntary termination of the Lease,
       the purchase of any Units by Union pursuant to Section 22.1 of
       the Lease or an ordinary Event of Loss under the Lease, on a
       Regular Distribution Date, (ii) in the case of prepayments with
       respect to a Multiple Loss under the Lease, a refunding or
       refinancing of the Equipment Notes or a purchase of the Equipment
       by Union from an Owner Trustee of a competitor Owner Participant,
       on any Business Day following 15 days notice from the Pass
       Through Trustee to The Depository Trust Company and (iii) in the
       case of payments received following a default in respect of any
       Equipment Note, on the second day of any month, and (iv) with
       respect to an ETC, the earliest day of a month for which it is
       practicable for the Pass Through Trustee to give notice pursuant
       to Section 4.2(c) 20 days prior thereto.

            "Special Payment" means (i) with respect to an Equipment
       Note (or portion thereof), any payment of principal of,
       Make-Whole Amount, if any, and interest on such Equipment Note
       (or portion thereof) resulting from the prepayment of such
       Equipment Note (or portion thereof) pursuant to Section 2.10 of
       the applicable Indenture, (ii) with respect to an Equipment Note,
       any payment of principal of and interest (including any interest
       accruing upon default) on, or any other amount in respect of,
       such Equipment Note upon an Indenture Default in respect thereof,
       including payments received on account of the purchase by the
       Owner Trustee of such Equipment Notes, (iii) the amounts required
       to be distributed pursuant to the fifth paragraph of Section
       2.1(b), (iv) any proceeds from the sale of any Equipment Note by
       the Pass Through Trustee pursuant to Article VI hereof; (v) with
       respect to an ETC, any payment of principal of and interest
       (including any interest accruing upon default) on, or any other
       amount in respect of, such ETC upon an Equipment Trust Default in
       respect thereof or upon an acceleration under the Equipment Trust
       Agreement relating to such ETC, or (vi) any proceeds from the
       sale of an ETC by the Pass Through Trustee pursuant to Article VI
       hereof; and "Special Payments" means all of such Special
       Payments.

            "Special Payments Account" means the account or accounts
       created and maintained pursuant to Section 4.1(b).

                                     -11-
 

   17



            "Specified Investments" means (i) direct obligations of the
       United States of America and agencies thereof for which the full
       faith and credit of the United States of America is pledged, (ii)
       obligations fully guaranteed by the United States of America,
       (iii) certificates of deposit issued by, or bankers' acceptances
       of, or time deposits (including overnight deposits) with, any
       bank, trust company or national banking association incorporated
       or doing business under the laws of the United States of America
       or one of the States thereof having combined capital and surplus
       and retained earnings of at least $500,000,000 (including the
       Indenture Trustee or Owner Trustee, in their respective
       individual capacities, if such conditions are met), (iv)
       commercial paper of companies, banks, trust companies or national
       banking associations incorporated or doing business under the
       laws of the United States of America or one of the States thereof
       and in each case having a rating of A-l/P-l or better assigned to
       such commercial paper by Standard & Poor's Corporation or Moody's
       Investors Service, Inc. (or, if neither such organization shall
       rate such commercial paper at any time, by any nationally
       recognized rating organization in the United States of America)
       and (v) repurchase agreements with any financial institution
       described in clause (iii) above having a combined capital and
       surplus of at least $750,000,000 fully collateralized by
       obligations of the type described in clauses (i) through (iv)
       above; provided, however, that if all of the above investments
       are unavailable, the entire amount to be invested may be used to
       purchase Federal Funds from an entity described in clause (iii)
       above; and provided further that no investment shall be eligible
       as a "Specified Investment" unless the final maturity or date of
       return of such investment occurs no later than _______.

            "Subsequent Cut-off Date" means __________.

            "Trust Indenture Act" means, except as otherwise provided in
       Section 9.6, the Trust Indenture Act of 1939 as in force at the
       date as of which this instrument was executed.

            "Trust Property" means the Union ETCs[, the Procor ETC,] the
       Guarantee and the Equipment Notes held as the property of the
       Pass Through Trust and all moneys at any time paid thereon and
       all moneys due and to become due thereunder, funds from time to
       time deposited in the Escrow Account (other than earnings on
       Specified Investments), the Certificate Account and the Special
       Payments Account and any proceeds from the sale by the Pass
       Through Trustee pursuant to Article VI hereof of any Equipment
       Note.

            "Trustee's Liens" means any Lien (within the meaning
       specified in Appendix A to the Participation Agreement) arising
       as a result of (a) any claim against the Pass Through Trustee or
       ____________, in its individual capacity, respectively, not
       resulting from the actions contemplated by the Operative
       Agreements (within the meaning specified in Appendix A to the
       Participation Agreement), (b) any act or omission of the Pass
       Through Trustee or ___________, in its individual capacity,
       respectively, which is not required or permitted by the Operative
       Agreements or is in violation of any of the terms of the
       Operative Agreements, (c) any claim against the Pass Through
       Trustee or _____________, in its individual capacity,
       respectively, with respect to Taxes (within the meaning specified
       in Appendix A to the Participation Agreement) or Transaction
       Costs


                                     -12-

   18


       (within the meaning specified in Appendix A to the Participation
       Agreement) against which Union is not required to indemnify the
       Pass Through Trustee, ______________, in its individual capacity,
       pursuant to the Participation Agreement or (d) any claim arising
       out of any transfer by the Pass Through Trustee, of all or any
       portion of its interests in the Equipment, the Trust Property or
       the Operative Agreements other than the transfer of title to or
       possession of any Equipment by the Pass Through Trustee pursuant
       to and in accordance with the applicable Equipment Trust
       Agreement or the Indenture or pursuant to the exercise of the
       remedies set forth in the Lease; provided, however, that any
       Trustee Lien which is attributable solely to the Pass Through
       Trustee or ____________, in its individual capacity and would
       otherwise constitute a Trustee Lien shall not constitute a
       Trustee Lien so long as (i) the existence of such Trustee Lien
       poses no material risk of the sale, forfeiture or loss of any
       Unit or any interest therein, including Equipment Notes, (ii) the
       existence of such Trustee Lien does not interfere in any way with
       the quiet enjoyment of any Unit by Union, as lessee and (iii) the
       Pass Through Trustee or ____________, in its individual capacity,
       is diligently contesting such Trustee Lien by appropriate
       proceedings.

            "Unit" has the meaning assigned to that term in the Lease.

            "Union Equipment Trust Trustee" means ____________, in its
       capacity as trustee under the Union Equipment Trust Agreement,
       and any other Person which may from time to time be acting as
       Equipment Trust Trustee in accordance with the provisions of the
       Union Equipment Trust Agreement.

            SECTION 1.2.  Compliance Certificates and Opinions.  Upon
       any application or request by the Companies to the Pass Through
       Trustee to take any action under any provision of this Pass
       Through Trust Agreement, Union shall furnish to the Pass Through
       Trustee an Officer's Certificate stating that, in the opinion of
       the signers, all conditions precedent, if any, provided for in
       this Pass Through Trust Agreement relating to the proposed action
       have been complied with and an Opinion of Counsel stating that in
       the opinion of such counsel all such conditions precedent, if
       any, have been complied with, except that in the case of any such
       application or request as to which the furnishing of such
       documents is specifically required by any provision of this Pass
       Through Trust Agreement relating to such particular application
       or request, no additional certificate or opinion need be
       furnished.

            Every certificate or opinion with respect to compliance with
       a condition or covenant provided for in this Pass Through Trust
       Agreement (other than a certificate provided pursuant to Section
       8.4(d)) shall include:

                 (i)  a statement that each individual signing such
            certificate or opinion has read such covenant or condition
            and the definitions herein relating thereto;

                                     -13-
 

   19



                 (ii)  a brief statement as to the nature and scope of
            the examination or investigation upon which the statements
            or opinions contained in such certificate or opinion are
            used;

                 (iii)  a statement that, in the opinion of each such
            individual, he or she has made such examination or
            investigation as necessary to enable him or her to express
            an informed opinion as to whether or not such covenant or
            condition has been complied with; and

                 (iv)  a statement as to whether, in the opinion of each
            such individual, such condition or covenant has been
            complied with.

            SECTION 1.3.  Form of Documents Delivered to Pass Through
       Trustee.  In any case where several matters are required to be
       certified by, or covered by an opinion of, any specified Person,
       it is not necessary that all such matters be certified by, or
       covered by the opinion of, only one such Person, or that they be
       so certified or covered by only one document, but one such Person
       may certify or give an opinion with respect to some matters and
       one or more other such Persons as to other matters and any such
       Person may certify or give an opinion as to such matters in one
       or several documents.

            Any Opinion of Counsel stated to be based on the opinion of
       other counsel shall be accompanied by a copy of such other
       opinion.

            Where any Person is required to make, give or execute two or
       more applications, requests, consents, certificates, statements,
       opinions or other instruments under this Pass Through Trust
       Agreement, they may, but need not, be consolidated and form one
       instrument.

            SECTION 1.4.  Acts of Certificateholders.

            (a)  Any direction, consent, waiver or other action provided
       by this Pass Through Trust Agreement to be given or taken by
       Certificateholders may be embodied in and evidenced by one or
       more instruments of substantially similar tenor signed by such
       Certificateholders in person or by an agent duly appointed in
       writing; and, except as herein otherwise expressly provided, such
       action shall become effective when such instrument or instruments
       are delivered to the Pass Through Trustee and, where it is hereby
       expressly required, to the Companies, any Equipment Trust Trustee
       or the Indenture Trustee.  Such instrument or instrument (and the
       action embodied therein and evidenced thereby) are herein
       sometimes referred to as the "Act" of the Certificateholders
       signing such instrument or instruments.  Proof of execution of
       any such instrument or of a writing appointing any such agent
       shall be sufficient for any purpose of this Pass Through Trust
       Agreement and conclusive in favor of the Pass Through Trustee,
       the Companies, any Equipment Trust Trustee and the Indenture
       Trustee, if made in the manner provided in this Section.


                                     -14-

   20



            (b)  The fact and date of the execution by any Person of any
       such instrument or writing may be proved by the certificate of
       any notary public or other officer of any jurisdiction authorized
       to take acknowledgments of deeds or administer oaths that the
       Person executing such instrument acknowledged to him the
       execution thereof, or by an affidavit of a witness to such
       execution sworn to before any such notary or such other officer
       and where such execution is by an officer of a corporation or
       association or a member of a partnership, on behalf of such
       corporation, association or partnership, such certificate or
       affidavit shall also constitute sufficient proof of his
       authority.  The fact and date of the execution of any such
       instrument or writing, or the authority of the Person executing
       the same, may also be proved in any other reasonable manner which
       the Pass Through Trustee deems sufficient.  In the absence of bad
       faith on the part of the Pass Through Trustee, an Officer's
       Certificate may be deemed to be conclusively proved.

            (c)  In determining whether the Certificateholders of the
       requisite Fractional Undivided Interests of Certificates
       Outstanding have given any direction, consent or waiver (a
       "Direction"), under this Pass Through Trust Agreement,
       Certificates owned by either of the Companies, the Owner Trustee,
       the Owner Participant or any Affiliate of any such Person shall
       be disregarded and deemed not to be Outstanding under this Pass
       Through Trust Agreement for purposes of any such determination.
       In determining whether the Pass Through Trustee shall be
       protected in relying upon any such Direction, only Certificates
       which the Pass Through Trustee knows to be so owned shall be so
       disregarded.  Notwithstanding the foregoing, (i) if any such
       Person owns 100% of the Certificates Outstanding, such
       Certificates shall not be so disregarded as aforesaid, and (ii)
       if any amount of Certificates so owned by any such Person have
       been pledged in good faith, such Certificates shall not be
       disregarded as aforesaid if the pledgee establishes to the
       satisfaction of the Pass Through Trustee the pledgee's right so
       to act with respect to such Certificates and that the pledgee is
       not either of the Companies, the Owner Trustee, the Pass Through
       Trustee, the Owner Participant or any Affiliate of any such
       Person.

            (d)  Union, on behalf of the Companies, may at its option by
       delivery of an Officer's Certificate to the Pass Through Trustee
       set a record date to determine the Certificateholders entitled to
       give any consent, request, demand, authorization, direction,
       notice, waiver or other Act.  Notwithstanding Section 316(c) of
       the Trust Indenture Act, such record date shall be the record
       date specified in such Officer's Certificate which shall be a
       date not more than 30 nor less than 15 days prior to the first
       solicitation of Certificateholders in connection therewith.  If
       such a record date is fixed, such consent, request, demand,
       authorization, direction, notice, waiver or other Act may be
       given before or after such record  date, but only the
       Certificateholders of record at the close of business on such
       record date shall be deemed to be Certificateholders for the
       purposes of determining whether Certificateholders of the
       requisite proportion of Outstanding Certificates have authorized
       or agreed or consented to such consent, request, demand,
       authorization, direction, notice, waiver or other Act, and for
       that purpose the Outstanding Certificates shall be computed as of
       such record date; provided that no such consent, request, demand,
       authorization, direction, notice, waiver or other act by the
       Certificateholders on such

                                     -15-
 

   21


       record date shall be deemed effective unless it shall become
       effective pursuant to the provisions of this Agreement not later
       than one year after the record date.

            (e)  Any direction, consent, waiver or other action by the
       Certificateholder of any Certificate shall bind the
       Certificateholder of every Certificate issued upon the transfer
       thereof or in exchange thereof or in lieu thereof, whether or not
       notation of such action is made upon such Certificate.

            (f)  Except as otherwise provided in Section 1.4(c),
       Certificates owned by or pledged to any Person shall have an
       equal and proportionate benefit under the provisions of this Pass
       Through Trust Agreement, without preference, priority, or
       distinction as among all of the Certificates.


                                   ARTICLE II

                    ACQUISITION OF EQUIPMENT NOTES AND ETCs;
                       ORIGINAL ISSUANCE OF CERTIFICATES

            SECTION 2.1.  Issuance of Certificates; Acquisition of
       Equipment Notes and ETCs.

            (a)  Upon request of Union, the Pass Through Trustee shall
       execute, authenticate and deliver Certificates with an aggregate
       principal amount equalling the aggregate principal amount of the
       Equipment Notes and ETCs, as the case may be, to be purchased by
       the Pass Through Trustee pursuant to the Equipment Trust
       Agreements, this Pass Through Trust Agreement and the
       Participation Agreement and evidencing the entire ownership of
       the Pass Through Trust.  The Pass Through Trustee, not in its
       individual capacity but solely as Pass Through Trustee, shall
       issue and sell such Certificates, in authorized denominations and
       in such Fractional Undivided Interests, so as to result in the
       receipt of consideration in an amount equal to the aggregate
       principal amount of the Equipment Notes and ETCs to be issued
       from time to time, as contemplated by the Participation Agreement
       and the Equipment Trust Agreements, respectively, and,
       concurrently therewith, the Pass Through Trustee shall purchase,
       in the case of the Equipment Notes pursuant to the terms and
       conditions of the Participation Agreement, the Equipment Notes
       (except Postponed Notes, if any) then issued and, in the case of
       the ETCs pursuant to the terms and conditions of the Union
       Equipment Trust Agreement, the ETCs (except Postponed Notes, if
       any) then issued, at a purchase price equal to the amount of
       consideration received by the Pass Through Trustee with respect
       to such Equipment Notes and ETCs, respectively.  Upon the
       issuance after the Issuance Date of any Equipment Notes and Union
       ETCs not previously purchased by the Pass Through Trustee, the
       Pass Through Trustee shall purchase at such time such Equipment
       Notes and Union ETCs at a purchase price equal to the amount of
       consideration received by the Pass Through Trustee with respect
       to such Equipment Notes and Union ETCs.  Except as provided in
       Sections 3.4 and 3.5 hereof, the Pass Through Trustee shall


                                     -16-

   22


       not execute, authenticate or deliver Certificates in excess of
       the aggregate amount specified in this paragraph.  The provisions
       of this Subsection (a) are subject to the provisions of
       Subsections (b) and (c) below.

            (b)  If Union shall deliver to the Pass Through Trustee on
       or prior to the Issuance Date a Postponement Notice, the Pass
       Through Trustee shall postpone the purchase of the Equipment
       Notes or ETCs (the "Postponed Notes") referred to in such
       Postponement Notice and shall promptly deposit into an escrow
       account (the "Escrow  Account") an amount equal to the purchase
       price of such Postponed Notes (the "Escrowed Funds").  The
       Escrowed Funds so deposited into the Escrow Account shall be
       invested by the Pass Through Trustee at the written direction and
       risk of, and for the benefit of, Union in Specified Investments
       maturing (i) no later than the Initial Cut-off Date or (ii) if
       Union has given notice to the Pass Through Trustee that the
       Postponed Notes will not be issued, on the next Special
       Distribution Date, if such investments are reasonably available
       for purchase.  The Pass Through Trustee shall make withdrawals
       from the Escrow Account only as provided in this Pass Through
       Trust Agreement.  An account statement delivered by the Pass
       Through Trustee to Union shall be deemed written confirmation by
       Union that the investment transactions identified therein
       accurately reflect the investment directions given to the Pass
       Through Trustee by Union, unless Union notifies the Pass Through
       Trustee in writing to the contrary within thirty (30) days of
       receipt of such statement.

            The Pass Through Trustee shall hold all Specified
       Investments until the maturity thereof and will not sell or
       otherwise transfer such Specified Investments.  If Specified
       Investments held in the Escrow Account mature prior to the
       Initial Cut-off Date, any proceeds received on the maturity of
       such Specified Investments (other than any earnings thereon)
       shall be reinvested by the Pass Through Trustee at the written
       direction and risk of, and for the benefit of, Union in Specified
       Investments maturing (i) no later than the Initial Cut-off Date
       or (ii) if Union has given notice to the Pass Through Trustee
       that the Postponed Notes will not be issued, on the next Special
       Distribution Date, if such investments are reasonably available
       for purchase.

            Any earnings on Specified Investments received from time to
       time by the Pass Through Trustee shall be promptly distributed to
       Union to an account specified in writing by Union.  Union shall
       pay to the Pass Through Trustee for deposit to the Escrow Account
       an amount equal to any losses on Specified Investments as
       incurred.

            On or prior to the Initial Cut-off Date, upon the written
       request of Union and the satisfaction of the closing conditions
       specified in the Participation Agreement and the Union Equipment
       Trust Agreement, as the case may be, the Pass Through Trustee
       shall purchase the Postponed Notes with the Escrowed Funds.  The
       purchase price shall equal the principal amount of such Postponed
       Notes.

            If any of the Postponed Notes will not be issued on or prior
       to the Initial Cut-off Date for any reason, Union shall so notify
       the Pass Through Trustee and Union shall pay to the Pass

                                     -17-
 

   23


       Through Trustee on _______ for deposit in the Special Payments
       Account, in immediately available funds, an amount equal to the
       interest that would have accrued on such Postponed Notes at a
       rate or rates equal to the interest rate or rates applicable to
       the Certificates issued under the Pass Through Trust from the
       Issuance Date to, but not including, ______ and the Pass Through
       Trustee shall transfer an amount equal to that amount of Escrowed
       Funds that would have been used to purchase such Postponed Notes
       and the amount paid by Union under this paragraph to the Special
       Payments Account for distribution as a Special Payment in
       accordance with the provisions hereof.

            (c)  Notwithstanding the foregoing Subsection (b), with
       respect to amounts received by the Pass Through Trustee for the
       purchase of Equipment Notes and Union ETCs to be issued after the
       Initial Cut-off Date pursuant to the terms and conditions of the
       Participation Agreement or the Union Equipment Trust Agreement,
       the Pass Through Trustee shall promptly deposit into the Escrow
       Account an amount equal to the purchase price of such Equipment
       Notes or Union ETCs.  The Escrowed Funds so deposited into the
       Escrow Account shall be invested by the Pass Through Trustee at
       the written direction and risk of, and for the benefit of, Union
       in Specified Investments maturing (i) no later than the
       Subsequent Cut-off Date or (ii) if Union has given notice to the
       Pass Through Trustee that the aforementioned Equipment Notes or
       Union ETCs will not be issued, on the next Special Distribution
       Date, if such investments are reasonably available for purchase.
       The Pass Through Trustee shall make withdrawals from the Escrow
       Account only as provided in this Pass Through Trust Agreement.
       An account statement delivered by the Pass Through Trustee to
       Union shall be deemed written confirmation by Union that the
       investment transactions identified therein accurately reflect the
       investment directions given to the Pass Through Trustee by Union,
       unless Union notifies the Pass Through Trustee in writing to the
       contrary within thirty (30) days of receipt of such statement.

            The Pass Through Trustee shall hold all Specified
       Investments until the maturity thereof and will not sell or
       otherwise transfer such Specified Investments.  If Specified
       Investments held in the Escrow Account mature prior to the
       Subsequent Cut-off Date, any proceeds received on the maturity of
       such Specified Investments (other than any earnings thereon)
       shall be reinvested by the Pass Through Trustee at the written
       direction and risk of, and for the benefit of, Union in Specified
       Investments maturing (i) no later than the Subsequent Cut-off
       Date or (ii) if Union has given notice to the Pass Through
       Trustee that the aforementioned Equipment Notes or Union ETCs
       will not be issued, on the next Special Distribution Date, if
       such investments are reasonably available for purchase.

            Any earnings on Specified Investments received from time to
       time by the Pass Through Trustee shall be promptly distributed to
       Union to an account specified in writing by Union.  Union shall
       pay to the Pass Through Trustee for deposit to the Escrow Account
       an amount equal to any losses on Specified Investments as
       incurred.

                                     -18-
 

   24



            On or prior to the Subsequent Cut-off Date, upon the written
       request of Union and the satisfaction of the closing conditions
       specified in the Participation Agreement and the Union Equipment
       Trust Agreement, the Pass Through Trustee shall purchase the
       applicable Equipment Notes and Union ETCs with the Escrowed
       Funds.  The purchase price shall equal the principal amount of
       such Postponed Notes.

            If any of the applicable Equipment Notes or Union ETCs will
       not be issued on or prior to the Subsequent Cut-off Date for any
       reason, Union shall so notify the Pass Through Trustee and Union
       shall pay to the Pass Through Trustee on _______ for deposit in
       the Special Payments Account, in immediately available funds, an
       amount equal to the interest that would have accrued on such
       Equipment Notes or Union ETCs, as the case may be, at a rate or
       rates equal to the interest rate or rates applicable to the
       Certificates issued under the Pass Through Trust from the
       Issuance Date to, but not including, ________ and the Pass
       Through Trustee shall transfer an amount equal to that amount of
       Escrowed Funds that would have been used to purchase such
       Equipment Notes or Union ETCs and the amount paid by Union under
       this paragraph to the Special Payments Account for distribution
       as a Special Payment in accordance with the provisions hereof.

            SECTION 2.2.  Declaration of Trust; Acceptance By Pass
       Through Trustee.  The Pass Through Trustee, upon the execution
       and delivery of this Pass Through Trust Agreement, acknowledges
       its acceptance of all right, title, and interest in and to the
       Equipment Notes and the ETCs, as the case may be, acquired
       pursuant to Section 2.1 hereof and, in the case of the Equipment
       Notes, the Participation Agreement, and declares that the Pass
       Through Trustee holds and will hold such right, title, and
       interest, together with all other property constituting the Trust
       Property, for the benefit of all present and future
       Certificateholders, upon the trusts herein set forth.  By its
       payment for and acceptance of each Certificate issued to it
       hereunder, each initial Certificateholder as grantor of the Trust
       thereby joins in the creation and declaration of the Trust.

            SECTION 2.3.  Limitation of Powers.  The Pass Through Trust
       is constituted solely for the purpose of making the investment in
       the Equipment Notes and the ETCs, and, except as set forth
       herein, the Pass Through Trustee is not authorized or empowered
       to acquire any other investments or engage in any other
       activities and, in particular, the Pass Through Trustee is not
       authorized or empowered to do anything that would cause the Pass
       Through Trust to fail to qualify as a "grantor trust" for federal
       income tax purposes (including as subject to this restriction,
       acquiring any Equipment (as defined in the Indenture) by bidding
       the Equipment Notes, the ETCs or otherwise, or taking any action
       with respect to any such Equipment once acquired).

                                     -19-
 

   25



            SECTION 2.4.  Sale of Equipment Notes Under Certain
       Circumstances.  If Union elects, in accordance with Section 10.2
       of the Participation Agreement, to refund or refinance Equipment
       Notes, the Pass Through Trustee shall, upon satisfaction of the
       conditions set forth in Section 10.2 of the Participation
       Agreement, transfer such Equipment Notes to the Person designated
       by Union and will take any other action reasonably required to
       effect such refunding.


 


                                  ARTICLE III

                                THE CERTIFICATES

            SECTION 3.1.  Form, Denomination and Execution of
       Certificates.  The Certificates shall be issued in fully
       registered form without coupons and shall be substantially in the
       form attached hereto as Exhibit A, together with the Guarantee of
       Union attached thereto in substantially the form attached hereto
       as Exhibit B, with such omissions, variations and insertions as
       are permitted by this Pass Through Trust Agreement, and may have
       such letters, numbers or other marks of identification and such
       legends or endorsements printed, lithographed, typewritten or
       engraved thereon, as may be required to comply with the rules of
       any securities exchange on which the Certificates may be listed
       or to conform to any usage in respect thereof, or as may,
       consistently herewith, be prescribed by the Pass Through Trustee
       or by the officer executing such Certificates, such determination
       by such officer to be evidenced by his or her signing of the
       Certificates.

            Except as provided in Section 3.9, the definitive
       Certificates (the "Registered Certificates") shall be printed,
       lithographed, typewritten or engraved or produced by any
       combination of these methods or may be produced in any other
       manner permitted by the rules of any securities exchange on which
       the Certificates may be listed, all as determined by the officer
       executing such Certificates, as evidenced by his execution of
       such Certificates.

            The Certificates shall be issued in minimum denominations of
       $1,000 or any integral multiple of $1,000 in excess thereof,
       except that one Certificate may be issued in a denomination of
       less than $1,000 (the "De Minimis Certificate").

            The Certificates shall be executed on behalf of the Pass
       Through Trustee by manual or facsimile signature of a Responsible
       Officer of the Pass Through Trustee.  Certificates bearing the
       manual or facsimile signature of an individual who was, at the
       time when such signature was affixed, authorized to sign on
       behalf of the Pass Through Trustee shall be valid and binding
       obligations of the Pass Through Trustee, notwithstanding that
       such individual has ceased to be so authorized prior to the
       authentication and delivery of such Certificates or did not hold
       such office at the date of such Certificates.  No Certificate
       shall be entitled to any benefit under this Pass Through Trust
       Agreement, or be valid for any purpose, unless there appears on
       such Certificate a certificate of authentication substantially in
       the form set forth in Exhibit A hereto executed by the Pass
       Through Trustee by manual signature, and such certificate of
       authentication upon any


                                     -20-

   26


       Certificate shall be conclusive evidence, and the only evidence,
       that such Certificate has been duly authenticated and delivered
       hereunder.  All Certificates shall be dated the date of their
       authentication.

            SECTION 3.2.  Authentication of Certificates.  The Pass
       Through Trustee shall duly authenticate and deliver Certificates
       in authorized denominations equalling in the aggregate principal
       amount of the Equipment Notes and the ETCs, as the case may be,
       to be purchased by the Pass Through Trustee pursuant to the
       Equipment Trust Agreements, this Pass Through Trust Agreement and
       the Participation Agreement, and evidencing the entire ownership
       of the Trust.

            SECTION 3.3.  Temporary Certificates.  Pending the
       preparation of Registered Certificates, the Pass Through Trustee
       may execute, authenticate and deliver temporary Certificates
       which are printed, lithographed, typewritten, or otherwise
       produced, in any denomination, containing substantially the same
       terms and provisions as set forth in Exhibit A, except for such
       appropriate insertions, omissions, substitutions and other
       variations relating to their temporary nature as the officer
       executing such temporary Certificates may determine, as evidenced
       by their execution of such temporary Certificates.

            The Companies will cause Registered Certificates to be
       prepared without unreasonable delay after such Registered
       Certificates are required to be issued pursuant to Section
       3.9(d).  After the preparation of Registered Certificates, the
       temporary Certificates shall be exchangeable for Registered
       Certificates upon surrender of the temporary Certificates at the
       Corporate Trust Office of the Pass Through Trustee, or at the
       office or agency of the Pass Through Trustee maintained in
       accordance with Section 7.12, without charge to the holder.  Upon
       surrender for cancellation of any one or more temporary
       Certificates, the Pass Through Trustee shall execute,
       authenticate and deliver in exchange therefor Registered
       Certificates of authorized denominations of a like aggregate
       Fractional Undivided Interest; shall destroy any such temporary
       Certificate(s) theretofore delivered and exchanged; and shall
       certify, in writing, to the Companies such destruction.  Until so
       exchanged, such temporary Certificates shall in all respects be
       entitled to the same benefits under this Pass Through Trust
       Agreement as Registered Certificates.

            SECTION 3.4.  Registration of Transfer and Exchange of
       Certificates.  The Pass Through Trustee shall cause to be kept at
       the Corporate Trust Office or the office or agency to be
       maintained by it in accordance with the provisions of Section
       7.12 a register (the "Register") in which, subject to such
       reasonable regulations as it may prescribe, the Pass Through
       Trustee shall provide for the registration of Certificates and of
       transfers and exchanges of Certificates as herein provided.  The
       Pass Through Trustee shall initially be the registrar (the
       "Registrar") for the purpose of registering Certificates and
       transfers and exchanges of Certificates as herein provided.

            Upon surrender for registration of transfer of any
       Certificate at the Corporate Trust Office or such other office or
       agency, the Pass Through Trustee shall execute, authenticate and
       deliver,


                                     -21-

   27


       in the name of the designated transferee or transferees, one or
       more new Certificates in authorized denominations of a like
       aggregate Fractional Undivided Interest.
            At the option of a Certificateholder, Certificates may be
       exchanged for other Certificates of authorized denominations of a
       like aggregate Fractional Undivided Interest, upon surrender of
       the Certificates to be exchanged at any such office or agency.
       Whenever any Certificates are so surrendered for exchange, the
       Pass Through Trustee shall execute, authenticate and deliver the
       Certificates that the Certificateholder making the exchange is
       entitled to receive.  Every Certificate presented or surrendered
       for registration of transfer or exchange shall be duly endorsed
       or accompanied by a written instrument of transfer in form
       satisfactory to the Pass Through Trustee and the Registrar duly
       executed by the Certificateholder thereof or its attorney duly
       authorized in writing.

            No service charge shall be made to a Certificateholder for
       any registration of transfer or exchange of Certificates, but the
       Pass Through Trustee shall require payment of a sum sufficient to
       cover any tax or governmental charge that may be imposed in
       connection with any transfer or exchange of Certificates.

            All Certificates surrendered for registration of transfer
       and exchange shall be canceled and subsequently destroyed by the
       Pass Through Trustee.  The Pass Through Trustee shall provide the
       Companies with written certification as to the destruction of all
       such Certificates.

            SECTION 3.5.  Mutilated, Destroyed, Lost or Stolen
       Certificates.  If (a) any mutilated Certificate is surrendered to
       the Registrar, or the Registrar receives evidence to its
       satisfaction of the destruction, loss or theft of any
       Certificate, and (b) there is delivered to the Registrar and the
       Pass Through Trustee such security, indemnity or bond, as may be
       required by them to save each of them harmless, then, in the
       absence of notice to the Registrar or the Pass Through Trustee
       that such Certificate has been acquired by a bona fide purchaser,
       the Pass Through Trustee shall execute, authenticate and deliver,
       in exchange for or in lieu of any such mutilated, destroyed, lost
       or stolen Certificate, a new Certificate of like Fractional
       Undivided Interest with the same final Regular Distribution Date.
       In connection with the issuance of any new Certificate under
       this Section 3.5, the Pass Through Trustee shall require the
       payment of a sum sufficient to cover any tax or other
       governmental charge that may be imposed in relation thereto and
       any other expenses (including the fees and expenses of the Pass
       Through Trustee and the Registrar) connected therewith.  Any
       duplicate Certificate issued pursuant to this Section 3.5 shall
       constitute conclusive evidence of the appropriate Fractional
       Undivided Interest in the Pass Through Trust, as if originally
       issued, whether or not the lost, stolen or destroyed Certificate
       shall be found at any time.

            SECTION 3.6.  Persons Deemed Owners.  Prior to due
       presentation of a Certificate for registration of transfer, the
       Pass Through Trustee, the Registrar, and any Paying Agent of the
       Pass Through Trustee may treat the person in whose name any
       Certificate is registered as the

                                     -22-
 

   28


       owner of such Certificate for the purpose of receiving
       distributions pursuant to Section 4.2 and for all other purposes
       whatsoever, and neither the Pass Through Trustee, the Registrar,
       nor any Paying Agent of the Pass Through Trustee shall be
       affected by any notice to the contrary.

            SECTION 3.7.  Cancellation.  All Certificates surrendered
       for payment or transfer or exchange shall, if surrendered to any
       Person party hereto other than the Registrar, be delivered to the
       Registrar for cancellation.  No Certificates shall be
       authenticated in lieu of or in exchange for any Certificates
       canceled as provided in this Section, except as expressly
       permitted by this Pass Through Trust Agreement.  All canceled
       Certificates held by the Registrar shall be destroyed and a
       certification of their destruction delivered to the Pass Through
       Trustee and the Companies.

            SECTION 3.8.  Limitation of Liability for Payments.  All
       payments or distributions made to Certificateholders under this
       Pass Through Trust Agreement shall be made only from the Trust
       Property and only to the extent that the Pass Through Trustee
       shall have sufficient income or proceeds from the Trust Property
       to make such payments in accordance with the terms of Article IV
       of this Pass Through Trust Agreement.  Each Certificateholder, by
       its acceptance of a Certificate, agrees that it will look solely
       to the income and proceeds from the Trust Property to the extent
       available for distribution to the Certificateholder thereof as
       provided in this Pass Through Trust Agreement.

            SECTION 3.9.  Book-Entry and Registered Certificates.

            (a)  Except for the De Minimis Certificate (if any), the
       Certificates may be issued in the form of one or more typewritten
       Certificates representing the Book-Entry Certificates, to be
       delivered to The Depository Trust Company, the initial Clearing
       Agency, by, or on behalf of, the Certificate Owners.  In such
       case, the Certificates delivered to The Depository Trust Company
       shall initially be registered on the Register in the name of Cede
       & Co., the nominee of the initial Clearing Agency, and no
       Certificate Owner will receive a Registered Certificate
       representing such Certificate Owner's interest in the
       Certificates, except as provided above and in Subsection (d)
       below.  Except with respect to the De Minimis Certificate (if
       any), unless and until Registered Certificates have been issued
       pursuant to Subsection (d) below:

                 (i)  the provisions of this Section 3.9 shall be in
            full force and effect;

                 (ii)  the Companies, the Paying Agent, the Registrar
            and the Pass Through Trustee may deal with the Clearing
            Agency for all purposes (including the making of
            distributions on the Certificates) as the authorized
            representative of the Certificate Owners;

                 (iii)  to the extent that the provisions of this
            Section 3.9 conflict with any other provisions of this Pass
            Through Trust Agreement, the provisions of this Section 3.9
            shall control;


                                     -23-

   29



                 (iv)  the rights of Certificate Owners shall be
            exercised only through the Clearing Agency and shall be
            limited to those established by law and agreements between
            such Certificate Owners and the Clearing Agency
            Participants; and until Registered Certificates are issued
            pursuant to Subsection (d) below, the Clearing Agency will
            make book-entry transfers among the Clearing Agency
            Participants and receive and transmit distributions of
            principal and interest and Make-Whole Amount, if any, on the
            Certificates to such Clearing Agency Participants; and

                 (v)  whenever this Pass Through Trust Agreement
            requires or permits actions to be taken based upon
            instructions or directions of Certificateholders holding
            Certificates evidencing a specified percentage of the
            Fractional Undivided Interests, the Clearing Agency shall be
            deemed to represent such percentage only to the extent that
            it has received instructions to such effect from Certificate
            Owners and/or Clearing Agency Participants owning or
            representing, respectively, such required percentage of the
            beneficial interest in Certificates and has delivered such
            instructions to the Pass Through Trustee.  The Pass Through
            Trustee shall have no obligation to determine whether the
            Clearing Agency has in fact received any such instructions.

            (b)  Except with respect to the De Minimis Certificate (if
       any), whenever notice or other communication to the
       Certificateholders is required under this Pass Through Trust
       Agreement, unless and until Registered Certificates shall have
       been issued pursuant to Subsection (d) below, the Pass Through
       Trustee shall give all such notices and communications specified
       herein to be given to Certificateholders to the Clearing Agency
       and/or the Clearing Agency Participants, and shall make available
       additional copies as requested by such Clearing Agency
       Participants.

            (c)  Unless and until Registered Certificates are issued
       pursuant to Subsection (d) below, on the Record Date prior to
       each Regular Distribution Date and Special Distribution Date, the
       Pass Through Trustee will request from the Clearing Agency a
       Securities Position Listing setting forth the names of all
       Clearing Agency Participants reflected on the Clearing Agency's
       books as holding interests in the Certificates on such Record
       Date.  The Pass Through Trustee will mail to each such Clearing
       Agency Participant the statements described in Section 4.3
       hereof.

            (d)  If (i) Union, on behalf of the Companies, advises the
       Pass Through Trustee in writing that the Clearing Agency is no
       longer willing or able to properly discharge its responsibilities
       and the Pass Through Trustee or Union, on behalf of the Companies
       is unable to locate a qualified successor, (ii) Union, on behalf
       of the Companies, at its option, advises the Pass Through Trustee
       in writing that it elects to terminate the book-entry system
       through the Clearing Agency or (iii) after the occurrence of an
       Event of Default, Certificate Owners of Book-Entry Certificates
       evidencing Fractional Undivided Interests aggregating not less
       than a majority in interest in the Pass Through Trust, by act of
       such Certificate Owners delivered to the Companies, and the Pass
       Through Trustee, advise the Pass Through Trustee and the Clearing
       Agency through the Clearing Agency Participants in writing that
       the continuation of a book-entry system through the Clearing


                                     -24-

   30


       Agency is no longer in the best interests of the Certificate
       Owners, then the Pass Through Trustee shall notify all
       Certificate Owners, through the Clearing Agency, of the
       occurrence of any such event and of the availability of
       Registered Certificates.  Upon surrender to the Pass Through
       Trustee of all the Certificates held by the Clearing Agency,
       accompanied by registration instructions from the Clearing Agency
       for registration of Registered Certificates in the names of
       Certificate Owners, the Pass Through Trustee shall issue and
       deliver the Registered Certificates in accordance with the
       instructions of the Clearing Agency.  Upon the issuance of
       Registered Certificates pursuant to this Section 3.9(d), the
       Record Date with respect to such Registered Certificates shall be
       the fifth Business Day preceding a Regular Distribution Date or
       Special Distribution Date, as applicable.  Neither the Companies,
       the Registrar, the Paying Agent nor the Pass Through Trustee
       shall be liable for any delay in delivery of such instructions
       and may conclusively rely on, and shall be protected in relying
       on, such registration instructions.  Upon the issuance of
       Registered Certificates, the Pass Through Trustee shall recognize
       the Persons in whose name the Registered Certificates are
       registered as the Certificateholders hereunder.  Neither the
       Companies nor the Pass Through Trustee shall be liable if the
       Pass Through Trustee or Union, on behalf of the Companies is
       unable to locate a qualified successor Clearing Agency.

            (e)  The Pass Through Trustee shall enter into the Letter of
       Representations and fulfill its responsibilities thereunder.


                                   ARTICLE IV

                          DISTRIBUTIONS; STATEMENTS TO
                               CERTIFICATEHOLDERS

            SECTION 4.1.  Certificate Account and Special Payments
       Account.

            (a)  The Pass Through Trustee shall establish and maintain
       on behalf of the Certificateholders the Certificate Account as
       one or more accounts, which shall be non-interest bearing
       accounts.  The Pass Through Trustee shall hold the Certificate
       Account in trust for the benefit of the Certificateholders, and
       shall make or permit withdrawals therefrom only as provided in
       this Pass Through Trust Agreement.  On each day when a Scheduled
       Payment is made under the Indenture or an Equipment Trust
       Agreement to the Pass Through Trustee, as holder of either the
       Equipment Notes or the ETCs issued under the Indenture or such
       Equipment Trust Agreement, as the case may be, the Pass Through
       Trustee upon receipt shall immediately deposit the aggregate
       amount of such Scheduled Payment in the Certificate Account.

            (b)  The Pass Through Trustee shall establish and maintain
       on behalf of the Certificateholders the Special Payments Account
       as one or more accounts, which shall be non-interest bearing
       except as provided in Section 4.4.  The Pass Through Trustee
       shall hold the Special Payments Account in trust for the benefit
       of the Certificateholders, and shall make or


                                     -25-
 

   31


       permit withdrawals therefrom only as provided in this Pass
       Through Trust Agreement.  On each day when one or more Special
       Payments (other than a Special Payment that represents the
       proceeds of any sale pursuant to Article VI by the Pass Through
       Trustee of an Equipment Note) are made under an Equipment Trust
       Agreement or the Indenture to the Pass Through Trustee, as holder
       of either the Equipment Notes or the ETCs issued under the
       Indenture or such Equipment Trust Agreement, as the case may be,
       the Pass Through Trustee upon receipt shall immediately deposit
       the aggregate amounts of such Special Payments in the Special
       Payments Account.  On the day Union makes the payment to the Pass
       Through Trustee described in the fifth paragraph of Section
       2.1(b), the Pass Through Trustee upon receipt shall immediately
       deposit the aggregate amount thereof in the Certificate Account.
       Upon the sale of any Equipment Note or ETC by the Pass Through
       Trustee pursuant to Article VI and the realization of any
       proceeds thereof, the Pass Through Trustee shall deposit the
       aggregate amount of such proceeds as a Special Payment in the
       Special Payments Account.

            (c)  The Pass Through Trustee shall present to the Indenture
       Trustee to which an Equipment Note relates such Equipment Note on
       the date of its stated final maturity, or in the case of any
       Equipment Note which is to be prepaid in whole pursuant to the
       relevant Indenture, on the applicable prepayment date under the
       Indenture.

            (d)  The Pass Through Trustee shall present to the Equipment
       Trust Trustee to which an ETC relates such ETC on the date of its
       stated final maturity.

            SECTION 4.2.  Distribution from Certificate Account and
       Special Payments Account.

            (a)  On each Regular Distribution Date or as soon thereafter
       as the Pass Through Trustee has confirmed receipt of the payment
       of the Scheduled Payments due with respect to the Equipment Notes
       or ETCs on such date, the Pass Through Trustee shall distribute
       to the Certificateholders of the Pass Through Trust out of the
       Certificate Account the entire amount deposited therein pursuant
       to Section 4.1(a).  There shall be so distributed to each
       Certificateholder of record on the Record Date with respect to
       such Regular Distribution Date (other than as provided in Section
       11.1 concerning the final distribution) by check mailed to such
       Certificateholder at the address appearing in the Register, such
       Certificateholder's pro rata share (based on the aggregate
       Fractional Undivided Interest held by such Certificateholder) of
       the aggregate amount in the Certificate Account; provided,
       however, that prior to the time that any Certificates are issued
       in the form of Registered Certificates, each Certificateholder
       may request in writing that such amounts be distributed by wire
       transfer of immediately available funds to an account specified
       by such Certificateholder.  If a Scheduled Payment is not
       received by the Pass Through Trustee on a Regular Distribution
       Date but is received on a later date, it shall be distributed on
       the date received in the manner described in the preceding
       sentence to the Certificateholders of record on the Record Date
       with respect to such Regular Distribution Date.

                                     -26-
 

   32



            (b)  On each Special Distribution Date with respect to any
       Special Payment or as soon thereafter as the Pass Through Trustee
       has confirmed receipt of the Special Payments due on the
       Equipment Notes or ETCs or realized upon the sale of any
       Equipment Note or ETCs, the Pass Through Trustee shall distribute
       out of the Special Payments Account the entire amount deposited
       therein pursuant to Section 4.1(b) of such Special Payment.
       There shall be so distributed to each Certificateholder of record
       on the Record Date with respect to such Special Distribution Date
       (other than as provided in Section 11.1 concerning the final
       distribution) by check mailed to such Certificateholder at the
       address appearing in the Register, such Certificateholder's pro
       rata share (based on the aggregate Fractional Undivided Interest
       held by such Certificateholder) of the aggregate amount in the
       Special Payments Account on account of such Special Payment;
       provided, however, that prior to the time that any Certificates
       are issued in the form of Registered Certificates, each
       Certificateholder may request in writing that such amount be
       distributed by wire transfer of immediately available funds to an
       account specified by such Certificateholder.

            (c)  The Pass Through Trustee shall at the expense of the
       Companies cause notice of each Special Payment to be mailed to
       each Certificateholder at his address as it appears in the
       Register.  In the case of a Special Payment other than pursuant
       to the fifth paragraph of Section 2.1(b) or the fifth paragraph
       of Section 2.1(c), such notice shall be mailed not less than 15
       days prior to the date any such Special Payment is scheduled to
       be distributed.  In the case of a Special Payment pursuant to the
       fifth paragraph of Section 2.1(b) or the fifth paragraph of
       Section 2.1(c) such notice should be given as promptly as
       practicable after the receipt of notice from the Companies that
       Postponed Notes or the Equipment Notes or Union ETCs referred to
       in Section 2.1(c) will not be issued.  In the case of any other
       Special Payments, such notice shall be mailed as soon as
       practicable after the Pass Through Trustee has confirmed that it
       has received funds for such Special Payment.  Notices mailed by
       the Pass Through Trustee shall set forth:

                 (i)  the Special Distribution Date and the Record Date
            therefor (except as otherwise provided in Section 11.1),

                 (ii)  the amount of the Special Payment for each $1,000
            face amount Certificate (taking into account any payment to
            be made by the Companies pursuant to Section 2.1(b) or (c),
            as applicable) and the amount thereof constituting
            principal, Make-Whole Amount, if any, and interest,

                 (iii)  the reason for the Special Payment, and

                 (iv)  if the Special Distribution Date is the same date
            as a Regular Distribution Date, the total amount to be
            received on such date for each $1,000 face amount
            Certificate.

            If the amount of Make-Whole Amount, if any, payable upon the
       prepayment of an Equipment Note has not been calculated at the
       time that the Pass Through Trustee mails notice


                                     -27-

   33

       of a Special Payment, it shall be sufficient if the notice sets
       forth the other amounts to be distributed and states that any
       Make-Whole Amount received will also be distributed.

            If any prepayment of the Equipment Notes is canceled, the
       Pass Through Trustee, as soon as possible after learning thereof,
       shall cause notice thereof to be mailed to each Certificateholder
       at its address as it appears on the Register.

            SECTION 4.3.  Statements to Certificateholders.

            (a)  On each Regular Distribution Date and Special
       Distribution Date, the Pass Through Trustee will include with
       each distribution of a Scheduled Payment or a Special Payment to
       Certificateholders a statement, giving effect to such
       distribution to be made on such Regular Distribution Date or
       Special Distribution Date, as the case may be, setting forth the
       following information (for each $1,000 face amount Certificate as
       to (i) and (ii) below):

                 (i)  The amount of such distribution allocable to
            principal and the amount allocable to Make-Whole Amount, if
            any;

                 (ii)  The amount of such distribution allocable to
            interest; and

                 (iii)  The Pool Balance and the Pool Factor.

            (b)  Within a reasonable period of time after the end of
       each calendar year but not later than the latest date permitted
       by law, the Pass Through Trustee shall furnish to each Person who
       at any time during such calendar year was a Certificateholder of
       record a statement containing the sum of the amounts determined
       pursuant to clauses (a)(i) and (a)(ii) of this Section 4.3 with
       respect to the Pass Through Trust for such calendar year or, in
       the event such Person was a Certificateholder of record during a
       portion of such calendar year, for the applicable portion of such
       year, and such other items as are readily available to the Pass
       Through Trustee and which a Certificateholder shall reasonably
       request as necessary for the purpose of such Certificateholder's
       preparation of its Federal income tax returns.  Such report and
       such other items shall be prepared on the basis of information
       supplied to the Pass Through Trustee by the Clearing Agency
       Participants and shall be delivered by the Pass Through Trustee
       to such Clearing Agency Participants to be available for
       forwarding by such Clearing Agency Participants to Certificate
       Owners.

            SECTION 4.4.  Investment of Special Payment Money.  Any
       money received by the Pass Through Trustee pursuant to Section
       4.1(b) representing a Special Payment which is not to be promptly
       distributed shall, to the extent practicable, be invested in
       Permitted Investments by the Pass Through Trustee pending
       distribution of such Special Payment pursuant to Section 4.2.
       Any investment made pursuant to this Section 4.4 shall be in such
       Permitted Investments having maturities not later than the date
       that such moneys are required to be used to make the payment

                                     -28-
 

   34


       required under Section 4.2 on the applicable Special Distribution
       Date and the Pass Through Trustee shall hold any such Permitted
       Investments until maturity.  The Pass Through Trustee shall have
       no liability with respect to any investment made pursuant to this
       Section 4.4, other than by reason of the willful misconduct or
       gross negligence of the Pass Through Trustee.  All income and
       earnings from such investments shall be distributed to the
       Certificateholders on such Special Distribution Date as part of
       such Special Payment.


                                   ARTICLE V

                                 THE COMPANIES

            SECTION 5.1.  Maintenance of Corporate Existence.  Each of
       the Companies, at its own cost and expense, will do or cause to
       be done all things necessary to preserve and keep in full force
       and effect its corporate existence, rights and franchises, except
       as otherwise specifically permitted in Section 5.2; provided,
       however, that neither of the Companies shall be required to
       preserve any right or franchise if it shall determine that the
       preservation thereof is no longer desirable in the conduct of its
       business.

            SECTION 5.2.  Consolidation, Merger or Sale of Assets
       Permitted.

            (a)  Union shall not consolidate with or merge into any
       other corporation or sell or convey all or substantially all of
       its assets to any Person[, nor shall Union permit Procor to
       consolidate with or merge into any other corporation or sell or
       convey all or substantially all of its assets to any Person
       (other than an Affiliate of Union)], unless the corporation
       formed by such consolidation or into which Union [or Procor, as
       applicable,] is merged or the Person which acquires by sale or
       conveyance all or substantially all of the assets of Union or
       Procor, as applicable, shall be a corporation organized and
       existing under the laws of the United States of America or any
       State or the District of Columbia [or, in the case of Procor any
       province of Canada], and shall execute and deliver to the Pass
       Through Trustee an agreement in form reasonably satisfactory to
       the Pass Through Trustee containing an assumption by such
       successor corporation of the due and punctual performance and
       observance of each covenant and condition of this Agreement to
       which Union [or Procor, as applicable,] is a party.

            (b)  Immediately after giving effect to such transaction, no
       Event of Default, no Indenture Default, no Equipment Trust
       Default, and no event that, after notice or lapse of time, or
       both, would become an Event of Default, an Indenture Default or
       an Equipment Trust Default, shall have occurred and be
       continuing.

            Upon any consolidation or merger, or any sale or conveyance,
       of all or substantially all of the assets of Union [or Procor, as
       the case may be], the successor corporation formed by such
       consolidation or into which Union [or Procor, as the case may
       be], is merged or to which such


                                     -29-

   35


       sale or conveyance is made shall succeed to, and be substituted
       for, and may exercise every right and power of, Union [or Procor,
       as the case may be], under this Agreement with the same effect as
       if such successor corporation had been named as Union [or Procor,
       as the case may be], herein.  No such sale or conveyance of all
       or substantially all of the assets of Union [or Procor, as the
       case may be], as an entirety shall have the effect of releasing
       Union [or Procor, as the case may be], or any successor
       corporation which shall theretofore have become such from its
       liability hereunder.

            (c)  The Pass Through Trustee may accept an Officer's
       Certificate and an Opinion of Counsel of Union, on behalf of the
       Companies, as conclusive evidence that any such consolidation,
       merger, sale, lease or conveyance, and any such assumption
       complies with the provisions of this Section 5.2.


                                   ARTICLE VI

                                    DEFAULT

            SECTION 6.1.  Events of Default.  If an Indenture Default
       under the Indenture or an Equipment Trust Default under either
       Equipment Trust Agreement (an "Event of Default") shall occur and
       be continuing, then, and in each and every case, so long as such
       Event of Default shall be continuing, the Pass Through Trustee
       may vote all of the Equipment Notes issued under the Indenture or
       ETCs issued under either of the Equipment Trust Agreements, as
       the case may be, to which such Event of Default relates that are
       held in the Trust Property, and upon the direction of the
       Certificateholders holding Certificates evidencing Fractional
       Undivided Interests aggregating not less than a majority in
       interest in the Pass Through Trust, the Trustee shall vote a
       corresponding majority of such Equipment Notes or such ETCs, as
       the case may be, in favor of directing the Indenture Trustee
       under the Indenture or the Equipment Trust Trustee under such
       Equipment Trust Agreement, as the case may be, to declare the
       unpaid principal amount of all Equipment Notes then outstanding
       under the Indenture or the ETCs then outstanding under such
       Equipment Trustee Agreement, as the case may be, and accrued
       interest thereon to be due and payable under, and in accordance
       with the provisions of, the Indenture or such Equipment Trust
       Agreement, as the case may be.  In addition, if an Indenture
       Default shall have occurred and be continuing under the Indenture
       or an Equipment Trust Default shall have occurred and be
       continuing under either Equipment Trust Agreement, the Pass
       Through Trustee may, and upon the direction of the
       Certificateholders holding Certificates evidencing Fractional
       Undivided Interests aggregating not less than a majority in
       interest in the Pass Through Trust, shall, vote all of the
       Equipment Notes issued under such Indenture or the ETCs issued
       under such Equipment Trust Agreement, as the case may be, to
       which such Event of Default relates that are held in the Pass
       Through Trust to direct such Indenture Trustee or such Equipment
       Trust Trustee, as the case may be, regarding the exercise of
       remedies provided in Article V of the Indenture or Article V of
       such Equipment Trust Agreement, as the case may be.  [In
       addition, if an Equipment Trust


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       Default has occurred and is continuing under the Procor ETC, the
       Pass Through Trustee in its own name, or as trustee of an express
       trust, or as attorney-in-fact for the Certificateholders, may,
       and upon direction of Certificateholders holding Certificates
       evidencing Fractional Undivided Interests aggregating not less
       than a majority in interest in the Pass Through Trust, shall,
       pursue such remedies as available to the Pass Through Trustee
       against Union under the Guarantee.]

            In addition, after an Event of Default shall have occurred
       and be continuing with respect to any Equipment Notes or ETCs,
       the Pass Through Trustee may in its discretion, and upon the
       direction of the Certificateholders holding Certificates
       evidencing Fractional Undivided Interests aggregating not less
       than a majority in interest in the Pass Through Trust shall, by
       such officer or agent as it may appoint, sell, convey, transfer
       and deliver (to the extent permitted by applicable law) all or
       part of such Equipment Notes or ETCs, without recourse to or
       warranty by the Pass Through Trustee or any Certificateholder, to
       any Person.  In any such case, the Pass Through Trustee shall
       sell, assign, contract to sell or otherwise dispose of and
       deliver such Equipment Note or Equipment Notes or ETC or ETCs in
       one or more parcels at public or private sale or sales, at any
       location or locations at the option of the Pass Through Trustee,
       all upon such terms and conditions as it may reasonably deem
       advisable and at such prices as it may reasonably deem advisable,
       for cash.  If the Pass Through Trustee so decides or is required
       to sell or otherwise dispose of any Equipment Note or ETC
       pursuant to this Section, the Pass Through Trustee shall take
       such of the actions described above as it may reasonably deem
       most effectual to complete the sale or other disposition of such
       Equipment Note or ETC, so as to provide for the payment in full
       of all amounts due on the Certificates.  [Notwithstanding the
       foregoing, any action taken by the Pass Through Trustee under
       this Section shall not, in the reasonable judgment of the Pass
       Through Trustee, be adverse to the best interests of the
       Certificateholders and the Pass Through Trustee shall give
       written notice to Procor of its proposed sale of the Procor ETC
       or any part thereof at least 20 days prior to the proposed sale.
       Such notice shall constitute an offer to Procor to sell the
       Procor ETC or such part thereof to Procor for a price equal to
       the outstanding principal amount thereof, plus all accrued and
       unpaid interest thereon, including interest at the Penalty Rate
       (as defined in the Procor Equipment Trust Agreement), if any,
       plus all other amounts due and owing thereunder and the costs and
       expenses incurred by the Pass Through Trustee, the
       Certificateholders and the Certificate Owners in connection with
       the collection thereof and enforcement of remedies relating
       thereto.  If prior to the expiration of such 20 day period,
       Procor pays the purchase price aforesaid to the Pass Through
       Trustee, the Pass Through Trustee shall transfer all of its
       right, title and interest in and to the Procor ETC or such part
       thereof to Procor.  If Procor does not pay such purchase price to
       the Pass Through Trustee prior to the expiration of such 20 day
       period, the Pass Through Trustee shall be free to sell the Procor
       ETC under this paragraph to any Person.  The Pass Through Trustee
       shall be required to give no more than one notice under this
       paragraph and Procor acknowledges that the failure of any
       proposed sale of the Procor ETC or part thereof shall not entitle
       Procor to receive any additional notice hereunder.]

            If, with respect to any Equipment Notes, an Indenture
       Default shall have occurred and be continuing and the Owner
       Trustee, which is a party to the Indenture has elected, in
       accordance


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       with the provisions of Section 5.04(b) of the Indenture, to
       purchase all of the then outstanding Equipment Notes issued under
       the Indenture at a price equal to the aggregate unpaid principal
       amount thereof, together with accrued interest thereon to the
       date of such purchase,  then the Pass Through Trustee shall sell
       to such Owner Trustee all of the Equipment Notes issued under the
       Indenture held in the Pass Through Trust at a price equal to the
       aggregate unpaid principal amount thereof, together with accrued
       interest thereon to the date of such sale.

            In no event shall (i) an Indenture Default constitute an
       Equipment Trust Default or (ii) an Equipment Trust Default
       constitute an Indenture Default.

            SECTION 6.2.  Incidents of Sale of Equipment Notes and ETCs.
       Upon any sale of all or any part of the Equipment Notes or ETCs,
       as the case may be, made either under the power of sale given
       under this Pass Through Trust Agreement or otherwise for the
       enforcement of this Pass Through Trust Agreement, the following
       shall be applicable:

                 (i)  Certificateholders and Pass Through Trustee May
            Purchase Equipment Notes or ETCs.  Any Certificateholder,
            the Pass Through Trustee in its individual or any other
            capacity or any other Person may bid for and purchase any of
            the Equipment Notes or ETCs, as the case may be, and upon
            compliance with the terms of sale, may hold, retain, possess
            and dispose of such Equipment Notes or ETCs, as the case may
            be, in their own absolute right without further
            accountability; provided, however, that this provision shall
            not apply to the sale of Equipment Notes or ETCs, as the
            case may be, pursuant to the last paragraph of Section 6.1.

                 (ii)  Receipt of Pass Through Trustee Shall Discharge
            Purchaser.  The receipt of the Pass Through Trustee or of
            the officer making such sale shall be a sufficient discharge
            to any purchaser for his purchase money, and, after paying
            such purchase money and receiving such receipt, such
            purchaser or its personal representative or assigns shall
            not be obliged to see to the application of such purchase
            money, or be in any way answerable for any loss,
            misapplication or non-application thereof.

                 (iii)  Application of Moneys Received Upon Sale.  Any
            moneys collected by the Pass Through Trustee upon any sale
            made either under the power of sale given by this Pass
            Through Trust Agreement or otherwise for the enforcement of
            this Pass Through Trust Agreement, shall be applied as
            provided in Section 4.2(b).

            SECTION 6.3.  Judicial Proceedings Instituted by Pass
       Through Trustee.
 
            (a)  Pass Through Trustee May Bring Suit.  If there shall be
       a failure to make payment of the principal of, Make-Whole Amount,
       if any, or interest on any Equipment Note or ETC, or if there
       shall be any failure to pay Rent (as defined in the Lease) under
       the Lease when due and payable, then the Pass Through Trustee, in
       its own name, and as trustee of an express trust, as


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       holder of such Equipment Notes or ETC, shall be, to the extent
       permitted by and in accordance with the terms of the Note
       Documents or ETC Documents, entitled and empowered to institute
       any suits, actions or proceedings at law, in equity or otherwise,
       for the collection of the sums so due and unpaid on such
       Equipment Notes or ETCs or under the Lease and may prosecute any
       such claim or proceeding to judgment or final decree with respect
       to the whole amount of any such sums so due and unpaid.

            (b)  Pass Through Trustee May File Proofs of Claim;
       Appointment of Pass Through Trustee as Attorney-in-Fact in
       Judicial Proceedings.  The Pass Through Trustee in its own name,
       or as trustee of an express trust, or as attorney-in-fact for the
       Certificateholders, or in any one or more of such capacities
       (irrespective of whether distributions on the Certificates shall
       then be due and payable, or the payment of the principal on the
       Equipment Notes or ETCs, as the case may be, shall then be due
       and payable, as therein expressed or by declaration or otherwise
       and irrespective of whether the Pass Through Trustee shall have
       made any demand to the Indenture Trustee for the payment of
       overdue principal, Make-Whole Amount (if any) or interest on
       Equipment Notes or ETCs, as the case may be), shall be entitled
       and empowered to file such proofs of claim and other papers or
       documents as may be necessary or advisable in order to have the
       claims of the Pass Through Trustee and of the Certificateholders
       allowed in any receivership, insolvency, bankruptcy, liquidation,
       readjustment, reorganization or any other judicial proceedings
       relative to the Companies, the Owner Trustee, the Owner
       Participant, the Equipment Trust Trustees or their respective
       creditors or property.  Any receiver, assignee, trustee,
       liquidator, sequestrator (or similar official) in any judicial
       proceeding is hereby authorized by each Certificateholder to make
       payments in respect of such claim to the Pass Through Trustee
       and, in the event that the Pass Through Trustee shall consent to
       the making of such payments directly to the Certificateholders,
       to pay to the Pass Through Trustee any amount due to it for the
       reasonable compensation, expenses, disbursements and advances of
       the Pass Through Trustee, its agents and counsel.  Nothing
       contained in this Pass Through Trust Agreement shall be deemed to
       give to the Pass Through Trustee any right to accept or consent
       to any plan of reorganization or otherwise by action of any
       character in any such proceeding to waive of change in any way
       any right of any Certificateholder.

            SECTION 6.4.  Control by Certificateholders.  Subject to
       Section 2.3, the Certificateholders holding Certificates
       evidencing Fractional Undivided Interests aggregating not less
       than a majority in interest in the Pass Through Trust shall have
       the right to direct the time, method and place of conducting any
       proceeding for any remedy available to the Pass Through Trustee,
       or exercising any trust or power conferred on the Pass Through
       Trustee under this Pass Through Trust Agreement, including any
       right of the Pass Through Trustee as holder of the Equipment
       Notes or ETCs, provided that:

                 (i)  such direction shall not be in conflict with any
            rule of law or with this Pass Through Trust Agreement and
            would not involve the Pass Through Trustee in personal
            liability or expense,


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   39



                 (ii)  the Pass Through Trustee shall not determine that
            the action so directed would be unjustly prejudicial to the
            Certificateholders not taking part in such direction,

                 (iii)  the Pass Through Trustee may take any other
            action deemed proper by the Pass Through Trustee which is
            not inconsistent with such direction, and

                 (iv)  if an Indenture Default under the Indenture or an
            Equipment Trust Default under either Equipment Trust
            Agreement shall have occurred and be continuing, such
            direction shall not obligate the Trustee to vote more than a
            corresponding majority of the related Equipment Notes or
            ETCs, as the case may be, held by the Trust in favor of
            directing any action by the Indenture Trustee with respect
            to such Indenture Default or by the Equipment Trust Trustee
            with respect to such Equipment Trust Default, as the case
            may be.

            SECTION 6.5.  Waiver of Past Defaults.  The
       Certificateholders holding Certificates evidencing Fractional
       Undivided Interests aggregating not less than a majority in
       interest in the Pass Through Trust may on behalf of all of the
       Certificateholders waive any past Event of Default hereunder and
       its consequences or may instruct the Pass Through Trustee to
       waive any past default under the Indenture, either Equipment
       Trust Agreement or this Agreement and its consequences, except a
       default (i) in the payment of the principal of, Make-Whole
       Amount, if any, or interest on any Equipment Notes or ETCs, as
       the case may be, or (ii) in respect of a covenant or provision
       hereof which under Article IX hereof cannot be modified or
       amended without the consent of the Certificateholder holding each
       Outstanding Certificate affected.

            Upon any such waiver, such default shall cease to exist with
       respect to this Pass Through Trust Agreement, and any Event of
       Default arising therefrom shall be deemed to have been cured for
       every purpose of this Pass Through Trust Agreement and any
       direction given by the Pass Through Trustee on behalf of such
       Certificateholders to the Indenture Trustee or Equipment Trust
       Trustee, as the case may be, shall be annulled with respect
       thereto; but no such waiver shall extend to any subsequent or
       other default or Event of Default or impair any right consequent
       thereon.  Upon any such waiver, the Pass Through Trustee shall
       vote all the Equipment Notes issued under the Indenture or ETCs
       issued under the relevant Equipment Trust Agreement, as the case
       may be, to waive the corresponding Default.

            SECTION 6.6.  Undertaking to Pay Court Costs.  All parties
       to this Pass Through Trust Agreement, and each Certificateholder
       by his acceptance of a Certificate, shall be deemed to have
       agreed that any court may in its discretion require, in any suit,
       action or proceeding for the enforcement of any right or remedy
       under this Pass Through Trust Agreement, or in any suit, action
       or proceeding against the Pass Through Trustee for any action
       taken or omitted by it as Pass Through Trustee hereunder, the
       filing by any party litigant in such suit, action or proceeding
       of an undertaking to pay the costs of such suit, action or
       proceeding, and that such court may, in its discretion, assess
       reasonable costs, including reasonable attorney's fees, against
       any party


                                     -34-

   40


       litigant in such suit, action or proceeding, having due regard to
       the merits and good faith of the claims or defenses made by such
       party litigant; provided, however, that the provisions of this
       Section shall not apply to (a) any suit, action or proceeding
       instituted by any Certificateholder or group of
       Certificateholders evidencing Fractional Undivided Interests
       aggregating more than 10% of the Pass Through Trust, (b) any
       suit, action or proceeding instituted by any Certificateholder
       for the enforcement of the distribution of payments pursuant to
       Section 4.2 hereof on or after the respective due dates expressed
       herein or (c) any suit, action or proceeding instituted by the
       Pass Through Trustee or as to which the Pass Through Trustee is
       made a party.

            SECTION 6.7.  Right of Certificateholders to Receive
       Payments Not to Be Impaired.  Anything in this Pass Through Trust
       Agreement to the contrary notwithstanding, including without
       limitation Section 6.8 hereof, the right of any Certificateholder
       to receive distributions of payments required pursuant to Section
       4.2 hereof on the Certificates when due, or to institute suit for
       the enforcement of any such payment on or after the applicable
       Regular Distribution Date or Special Distribution Date, shall not
       be impaired or affected without the consent of such
       Certificateholder.

            SECTION 6.8.  Certificateholders May Not Bring Suit Except
       Under Certain Conditions.  A Certificateholder shall not have the
       right to institute any suit, action or proceeding at law or in
       equity or otherwise with respect to this Pass Through Trust
       Agreement, for the appointment of a receiver or for the
       enforcement of any other remedy under this Pass Through Trust
       Agreement, unless:

                 (i)  such Certificateholder previously shall have given
            written notice to the Pass Through Trustee of a continuing
            Event of Default;

                 (ii)  the Certificateholders holding Certificates
            evidencing Fractional Undivided Interests aggregating not
            less than 25% of the Pass Through Trust shall have requested
            the Pass Through Trustee in writing to institute such
            action, suit or proceeding and shall have offered to the
            Pass Through Trustee indemnity as provided in Section
            7.3(v);

                 (iii)  the Pass Through Trustee shall have refused or
            neglected to institute any such action, suit or proceeding
            for 60 days after receipt of such notice, request and offer
            of indemnity; and

                 (iv)  no direction inconsistent with such written
            request has been given to the Pass Through Trustee during
            such 60-day period by the Certificateholders holding
            Certificates evidencing Fractional Undivided Interests
            aggregating not less than an a majority in interest in the
            Pass Through Trust.

            It is understood and intended that no one or more of the
       Certificateholders shall have any right in any manner whatever
       hereunder or under the Certificates to (x) surrender, impair,
       waive,

                                     -35-
 

   41


       affect, disturb or prejudice any property in the Trust Property
       or the lien of the Indenture or any Equipment Trust Agreement on
       any property subject thereto, or the rights of the
       Certificateholders or the holders of the Equipment Notes or ETCs,
       as the case may be, (y) obtain or seek to obtain priority over or
       preference to any other such Certificateholder or (z) enforce any
       right under this Pass Through Trust Agreement, except in the
       manner herein provided and for the equal, ratable and common
       benefit of all the Certificateholders subject to the provisions
       of this Pass Through Trust Agreement.

            SECTION 6.9.  Remedies Cumulative.  Every remedy given
       hereunder to the Pass Through Trustee or to any of the
       Certificateholders shall not be exclusive of any other remedy or
       remedies, and every such remedy shall be cumulative and in
       addition to every other remedy given hereunder or now or
       hereafter given by statute, law, equity or otherwise.


                                  ARTICLE VII

                            THE PASS THROUGH TRUSTEE

            SECTION 7.1.  Certain Duties and Responsibilities.

            (a)  Except during the continuance of an Event of Default,

                 (i)  the Pass Through Trustee undertakes to perform
            only such duties as are specifically set forth in this Pass
            Through Trust Agreement, and no implied covenants, duties or
            obligations shall be read into this Pass Through Trust
            Agreement against the Pass Through Trustee; and

                 (ii)  in the absence of bad faith on its part, the Pass
            Through Trustee may conclusively rely, as to the truth of
            the statements and the correctness of the opinions expressed
            therein, upon certificates or opinions furnished to the Pass
            Through Trustee and conforming to the requirements of this
            Pass Through Trust Agreement; but in the case of any such
            certificates or opinions which by any provision hereof are
            specifically required to be furnished to the Pass Through
            Trustee, the Pass Through Trustee shall be under a duty to
            examine the form of the same to determine whether or not
            they substantially conform to the requirements of this Pass
            Through Trust Agreement, but shall be under no duty to
            investigate the facts contained therein.

            (b)  In case an Event of Default has occurred and is
       continuing, the Pass Through Trustee shall exercise such of the
       right and powers vested in it by this Pass Through Trust
       Agreement, and use the same degree of care and skill in its
       exercise, as a prudent man would exercise or use under the
       circumstances in the conduct of his own affairs.


                                     -36-

   42



            (c)  No provision of this Pass Through Trust Agreement shall
       be construed to relieve the Pass Through Trustee from liability
       for its own negligent action, its own negligent failure to act,
       or its own willful misconduct, except that

                 (i)  this Subsection shall not be construed to limit
            the effect of Subsection (a) of this Section;

                 (ii)  the Pass Through Trustee shall not be liable for
            any error of judgement made in good faith by a Responsible
            Officer of the Pass Through Trustee;

                 (iii)  the Pass Through Trustee shall not be liable
            with respect to any action taken or omitted to be taken by
            it in good faith in accordance with the direction of the
            Certificateholders evidencing Fractional Undivided Interests
            aggregating not less than a majority in interest in the Pass
            Through Trust relating to the time, method and place of
            conducting any proceeding for any remedy available to the
            Pass Through Trustee, or exercising any trust or power
            conferred upon the Pass Through Trustee, under this Pass
            Through Trust Agreement; and

                 (iv)  no provision of this Pass Through Trust Agreement
            shall require the Pass Through Trustee to expend or risk its
            own funds in the performance of any of its duties hereunder,
            or in the exercise of any of its rights or powers, if it
            shall have reasonable grounds for believing that payment of
            such funds or adequate indemnity against such risk is not
            reasonably assured to it.

            (d)  Whether or not herein expressly so provided, every
       provision of this Pass Through Trust Agreement relating to the
       conduct or affecting the liability of or affording protection to
       the Pass Through Trustee shall be subject to the provisions of
       this Section.

            SECTION 7.2.  Notice of Defaults.  As promptly as
       practicable after, and in any event within 30 days after, the
       occurrence of any default (as such term is defined below)
       hereunder, the Pass Through Trustee shall transmit by mail to the
       Certificateholders, the Companies, the Owner Trustee, the
       Equipment Trust Trustees and the Indenture Trustees in accordance
       with Section 313(c) of the Trust Indenture Act, notice of such
       default hereunder known to the Pass Through Trustee, unless such
       default shall have been cured or waived; provided, however, that,
       except in the case of a default in the payment of the principal
       of (or Make-Whole Amount, if any) or interest on any Equipment
       Note or ETC, as the case may be, the Pass Through Trustee shall
       be protected in withholding such notice if and so long as the
       board of directors, the executive committee or a trust committee
       of directors and/or Responsible Officers of the Pass Through
       Trustee in good faith determine that the withholding of such
       notice is in the interests of the Certificateholders.  For the
       purpose of this Section, the term "default" means the occurrence
       of any Event of Default, except that in determining whether any
       such Event of Default has occurred

                                     -37-
 

   43


       for the purposes of this paragraph any grace period or notice in
       connection therewith shall be disregarded.

            SECTION 7.3.  Certain Rights of Trustee.  Except as
       otherwise provided in Section 7.1:

                 (i)  the Pass Through Trustee may rely and shall be
            protected in acting or refraining from acting in reliance
            upon any resolution, certificate, statement, instrument,
            opinion, report, notice, request, direction, consent, order,
            bond, debenture or other paper or document believed by it to
            be genuine and to have been signed or presented by the
            proper party or parties;

                 (ii)  any request or direction of the Companies
            mentioned herein shall be sufficiently evidenced by a
            Request;

                 (iii)  whenever in the administration of this Pass
            Through Trust Agreement the Pass Through Trustee shall deem
            it desirable that a matter be proved or established prior to
            taking, suffering or omitting any action hereunder, the Pass
            Through Trustee (unless other evidence be herein
            specifically prescribed) may, in the absence of bad faith on
            its part, rely upon an Officer's Certificate of Union, on
            behalf of the Companies, the Owner Trustee, the Equipment
            Trust Trustee or the Indenture Trustee;

                 (iv)  the Pass Through Trustee may consult with counsel
            and the advice of such counsel or any Opinion of Counsel
            shall be full and complete authorization and protection in
            respect of any action taken, suffered or omitted by it
            hereunder in good faith and in reliance thereon;

                 (v)  the Pass Through Trustee shall be under no
            obligation to exercise any of the rights or powers vested in
            it by this Pass Through Trust Agreement at the request or
            direction of any of the Certificateholders pursuant to this
            Pass Through Trust Agreement, unless such Certificateholders
            shall have offered to the Pass Through Trustee reasonable
            security or indemnity against the cost, expenses and
            liabilities which might be incurred by it in compliance with
            such request or direction;

                 (vi)  the Pass Through Trustee shall not be bound to
            make any investigation into the facts or matters stated in
            any resolution, certificate, statement, instrument, opinion,
            report, notice, request, direction, consent, order, bond,
            debenture or other paper or document;

                 (vii)  the Pass Through Trustee may execute any of the
            trusts or powers hereunder or perform any duties hereunder
            either directly or by or through agents or attorneys and the
            Pass Through Trustee shall not be responsible for any
            misconduct or negligence on the part of any agent or
            attorney appointed with due care by it hereunder; and


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                 (viii)  to the extent the provisions of this Section
            7.3 are inconsistent with the duties of the Pass Through
            Trustee as required by Section 315 of the Trust Indenture
            Act, the requirements of such Section 315 shall prevail.

            SECTION 7.4.  Not Responsible for Recitals or Issuance of
       Certificates.  The recitals contained herein and in the
       Certificates, except the certificates of authentication, shall
       not be taken as the statements of the Pass Through Trustee, and
       the Pass Through Trustee assumes no responsibility for their
       correctness.  Subject to Section 7.15, the Pass Through Trustee
       makes no representations as to the validity or sufficiency of
       this Pass Through Trust Agreement, any Note Document, any ETC
       Document, the Participation Agreement, the Equipment Notes, the
       ETCs or the Certificates, except that the Pass Through Trustee
       hereby represents and warrants that this Pass Through Trust
       Agreement has been, and each Certificate will be, executed and
       delivered by one of its officers who is duly authorized to
       execute and deliver such document on its behalf.

            SECTION 7.5.  May Hold Certificates.  The Pass Through
       Trustee, any Paying Agent, Registrar or any of their Affiliates
       or any other agent, in their respective individual or any other
       capacity, may become the owner or pledgee of Certificates and,
       subject to Sections 310(b) and 311 of the Trust Indenture Act,
       may otherwise deal with the Companies, the Owner Trustee, the
       Equipment Trust Trustees or the Indenture Trustee with the same
       rights it would have if it were not Pass Through Trustee, Paying
       Agent, Registrar or such other agent.

            SECTION 7.6.  Money Held in Pass Through Trust.  Money held
       by the Pass Through Trustee or the Paying Agent in trust
       hereunder need not be segregated from other funds except to the
       extent required herein or by law and neither the Pass Through
       Trustee nor the Paying Agent shall have any liability for
       interest upon any such moneys except as provided for herein.

            SECTION 7.7.  Compensation and Reimbursement.  The Companies
       agree, severally and not jointly:

                 (i)  to pay, or cause to be paid, pursuant to separate
            agreements between the Companies and the Pass Through
            Trustee to the Pass Through Trustee from time to time
            reasonable compensation for all services rendered by it
            hereunder (which compensation shall not be limited by any
            provision of law in regard to the compensation of a trustee
            of an express trust), except for the  Pass Through Trustee's
            initial fees, which fees shall be paid by the Owner Trustee;

                 (ii)  except as otherwise expressly provided herein, to
            reimburse, or cause to be reimbursed, the Pass Through
            Trustee upon its request for all reasonable out-of-pocket
            expenses, disbursements and advances incurred or made by the
            Pass Through Trustee in accordance with any provision of
            this Pass Through Trust Agreement (including the reasonable
            compensation and the expenses and disbursements of its
            agents and counsel), except any such expense, disbursement
            or advance as may be attributable to its negligence,


                                     -39-

   45


            willful misconduct or bad faith or as may be incurred due to
            the Pass Through Trustee's breach of its representations and
            warranties set forth in Sections 7.4 and 7.15 and except for
            the initial reasonable actual disbursements made by the Pass
            Through Trustee, which disbursements shall be reimbursed by
            the Owner Trustee;

                 (iii)  to indemnify, or cause to be indemnified, the
            Pass Through Trustee, in its individual and trust
            capacities, for, and to hold it harmless against, any loss,
            liability or expense (other than for or with respect to any
            tax) incurred without negligence, willful misconduct or bad
            faith, on its part, arising out of or in connection with the
            acceptance or administration of this Pass Through Trust,
            including the costs and expenses of defending itself against
            any claim or liability in connection with the exercise or
            performance of any of its powers or duties hereunder, except
            for any such loss, liability or expense incurred by reason
            of the Pass Through Trustee's breach of its representations
            and warranties set forth in Sections 7.4 and 7.15.  The Pass
            Through Trustee shall notify the Companies promptly of any
            claim for which it may seek indemnity.  The Companies shall
            defend the claim and the Pass Through Trustee shall
            cooperate in the defense.  The Pass Through Trustee may have
            separate counsel with the consent of the Companies, and the
            Companies will pay the reasonable fees and expenses of such
            counsel.  The Companies need not pay for any settlement made
            without their consent; and

                 (iv)  to indemnify, or cause to be indemnified, the
            Pass Through Trustee, solely in its individual capacity,
            for, and to hold it harmless against, any tax (other than
            for or with respect to any tax referred to in the next
            paragraph, provided that no indemnification shall be
            available with respect to any tax attributable to the Pass
            Through Trustee's compensation for serving as such) incurred
            without negligence, willful misconduct or bad faith, on its
            part, arising out of or in connection with the acceptance or
            administration of this Pass Through Trust, including any
            costs and expenses reasonably incurred in contesting the
            imposition of any such tax.  The Pass Through Trustee, in
            its individual capacity, shall notify the Companies promptly
            of any tax for which it may seek indemnity.  The Companies
            shall defend against the imposition of such tax and the Pass
            Through Trustee, in its individual capacity, shall cooperate
            in the defense.  The Pass Through Trustee, in its individual
            capacity, may have separate counsel with the consent of the
            Companies, and the Companies will pay the reasonable fees
            and expenses of such counsel.  The Companies need not pay
            for any taxes paid, in settlement or otherwise, without
            their consent.

            In addition, the Pass Through Trustee shall be entitled to
       reimbursement from, and shall have a lien prior to the
       Certificates upon, all property and funds held or collected by
       the Pass Through Trustee in its capacity as Pass Through Trustee
       for any tax incurred without negligence, bad faith or willful
       misconduct, on its part, arising out of or in connection with the
       acceptance or administration of this Pass Through Trust (other
       than any tax attributable to the Pass Through Trustee's
       compensation for serving as such), including any costs and
       expenses reasonably incurred


 
                                     -40-
   46


       in contesting the imposition of any such tax.  If the Pass
       Through Trustee reimburses itself for any such tax, it will
       within 30 days mail a brief report setting forth the
       circumstances thereof to all Certificateholders as their names
       and addresses appear in the Register.

            The Pass Through Trustee shall have a lien prior to the
       Certificates upon all property and funds held or collected by the
       Pass Through Trustee in its capacity as Pass Through Trustee.

            SECTION 7.8.  Corporate Trustee Required; Eligibility.  This
       Agreement shall at all times have a Pass Through Trustee which
       shall be eligible to act as a trustee under Section 310(a) of the
       Trust Indenture Act and shall have a combined capital and surplus
       of at least $100,000,000 and the obligations of which, whether
       now in existence or hereafter incurred, are fully and
       unconditionally guaranteed by a corporation organized and doing
       business under the laws of the United States of America, any
       State or Territory thereof or of the District of Columbia and
       having a combined capital and surplus of at least $100,000,000.
       If such corporation publishes reports of condition at least
       annually, pursuant to law or the requirements of Federal, State,
       Territorial or District of Columbia supervising or examining
       authority, then for the purposes of this Section 7.8, the
       combined capital and surplus of such corporation shall be deemed
       to be its combined capital and surplus as set forth in its most
       recent report of condition so published.

            The Pass Through Trustee shall comply with Section 310(b) of
       the Trust Indenture Act.
            In case at any time the Pass Through Trustee shall cease to
       be eligible in accordance with the provisions of this Section
       7.8, the Pass Through Trustee shall resign immediately in the
       manner and with the effect specified in Section 7.9.

            SECTION 7.9.  Resignation and Removal; Appointment of
       Successor.

            (a)  No resignation or removal of the Pass Through Trustee
       and no appointment of a successor Pass Through Trustee pursuant
       to this Article shall become effective until the acceptance of
       appointment by the successor Pass Through Trustee under Section
       7.10.

            (b)  The Pass Through Trustee may resign at any time by
       giving written notice thereof to the Companies, the Authorized
       Agents, the Owner Trustees, the Equipment Trust Trustees and the
       Indenture Trustee.  If an instrument of acceptance by a successor
       Pass Through Trustee shall not have been delivered to the
       Companies, the Owner Trustee, the Equipment Trust Trustees and
       the Indenture Trustee within 30 days after the giving of such
       notice of resignation, the resigning Pass Through Trustee may
       petition any court of competent jurisdiction for the appointment
       of a successor Pass Through Trustee.

            (c)  The Pass Through Trustee may be removed at any time by
       Act of the Certificateholders holding Certificates evidencing
       Fractional Undivided Interests aggregating not less than a
       majority in interest in the Pass Through Trust delivered to the
       Pass Through Trustee


                                     -41-

   47


       and to the Companies, the Owner Trustee, the Equipment Trustee
       Trustees and the Indenture Trustee.

            (d)  If at any time:

                 (i)  the Pass Through Trustee shall fail to comply with
            Section 310 of the Trust Indenture Act after written request
            therefor by the Companies or by any Certificateholder who
            has been a bona fide Certificateholder for at least six
            months; or

                 (ii)  the Pass Through Trustee shall cease to be
            eligible under Section 7.8 and shall fail to resign after
            written request therefor by the Companies or by any such
            Certificateholder; or

                 (iii)  the Pass Through Trustee shall become incapable
            of acting or shall be adjudged a bankrupt or insolvent or a
            receiver of the Pass Through Trustee or of its property
            shall be appointed or any public officer shall take charge
            or control of the Pass Through Trustee or of its property or
            affairs for the purpose of rehabilitation, conservation or
            liquidation;

       then, in any case, (x) the Companies may remove the Pass Through
       Trustee or (y) subject to Section 6.6, any Certificateholder who
       has been a bona fide Certificateholder for at least six months
       may, on behalf of himself and all others similarly situated,
       petition any court of competent jurisdiction for the removal of
       the Pass Through Trustee and the appointment of a successor Pass
       Through Trustee.

            (e)  If a Responsible Officer of the Pass Through Trustee
       shall obtain actual knowledge of an Avoidable Tax (as hereinafter
       defined) which has been or is likely to be asserted, the Pass
       Through Trustee shall promptly notify the Companies and the Owner
       Trustee thereof and shall, within 30 days of such notification,
       resign hereunder unless within such 30-day period the Pass
       Through Trustee shall have received notice that the Companies or
       the Owner Trustees have agreed to pay such tax.  The Companies
       shall promptly appoint a successor Pass Through Trustee in a
       jurisdiction where there are no Avoidable Taxes.  As used herein,
       an "Avoidable Tax" means a state or local tax: (i) upon (w) the
       Pass Through Trust, (x) the Trust Property, (y)
       Certificateholders or (z) the Pass Through Trustee for which the
       Pass Through Trustee is entitled to seek reimbursement from the
       Trust Property, and (ii) which would be avoided if the Pass
       Through Trustee were located in another state, or jurisdiction
       within a state, within the United States.  A tax shall not be an
       Avoidable Tax if the Companies or the Owner Trustee shall agree
       to pay, and shall pay, such tax.

            (f)  If the Pass Through Trustee shall resign, be removed or
       become incapable of acting, or if a vacancy shall occur in the
       office of the Pass Through Trustee for any cause, Union shall
       promptly appoint a successor Pass Through Trustee. If, within one
       year after such resignation,


                                     -42-

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       removal or incapacity, or the occurrence of such vacancy, a
       successor Pass Through Trustee shall be appointed by Act of the
       Certificateholders holding Certificates evidencing Fractional
       Undivided Interests aggregating not less than a majority in
       interest in the Pass Through Trust delivered to the Companies,
       the Owner Trustee, the Indenture Trustee, the Equipment Trust
       Trustees and the retiring Pass Through Trustee, the successor
       Pass Through Trustee so appointed shall, forthwith upon its
       acceptance of such appointment, become the successor Pass Through
       Trustee and supersede the retiring Pass Through Trustee.  If no
       successor Pass Through Trustee shall have been so appointed as
       provided above and accepted appointment in the manner hereinafter
       provided, any Certificateholder who has been a bona fide
       Certificateholder for at least six months may, on behalf of
       himself and all others similarly situated, petition any court of
       competent jurisdiction for the appointment of a successor Pass
       Through Trustee.

            (g)  The successor Pass Through Trustee shall give notice of
       the resignation and removal of the Pass Through Trustee and
       appointment of the successor Pass Through Trustee by mailing
       written notice of such event by first-class mail, postage
       prepaid, to the Certificateholders as their names and addresses
       appear in the Register.  Each notice shall include the name of
       such successor Pass Through Trustee and the address of its
       Corporate Trust Office.

            SECTION 7.10.  Acceptance of Appointment by Successor.
       Every successor Pass Through Trustee appointed hereunder shall
       execute, acknowledge and deliver to the Companies and the
       retiring Pass Through Trustee an instrument accepting such
       appointment, and thereupon the resignation or removal of the
       retiring Pass Through Trustee shall become effective and such
       successor Pass Through Trustee, without any further act, deed or
       conveyance, shall become vested with all the rights, powers,
       trusts and duties of the retiring Pass Through Trustee; but, on
       request of the Companies or the successor Pass Through Trustee,
       such retiring Pass Through Trustee shall execute and deliver an
       instrument transferring to such successor Pass Through Trustee
       all the rights, powers and trusts of the retiring Pass Through
       Trustee and shall duly assign, transfer and deliver to such
       successor Pass Through Trustee all property and money held by
       such retiring Pass Through Trustee hereunder, subject
       nevertheless to its lien, if any, provided for in Section 7.7.
       Upon request of any such successor Pass Through Trustee, Union,
       the retiring Pass Through Trustee and such successor Pass Through
       Trustee shall execute and deliver any and all instruments
       containing such provisions as shall be necessary or desirable to
       transfer and confirm to, and for more fully and certainly vesting
       in, such successor Pass Through Trustee all such rights, powers
       and trusts.

            No successor Pass Through Trustee shall accept its
       appointment unless at the time of such acceptance such successor
       Pass Through Trustee shall be qualified and eligible under this
       Article.

            SECTION 7.11.  Merger, Conversion, Consolidation or
       Succession to Business.  Any corporation into which the Pass
       Through Trustee may be merged or converted or with which it may
       be consolidated, or any corporation resulting from any merger,
       conversion or consolidation to which the Pass Through Trustee
       shall be a party, or any corporation succeeding to all or


                                     -43-

   49


       substantially all of the corporate trust business of the Pass
       Through Trustee, shall be the successor of the Pass Through
       Trustee hereunder, provided such corporation shall be otherwise
       qualified and eligible under this Article, without the execution
       or filing of any paper or any further act on the part of any of
       the parties hereto.  In case any Certificates shall have been
       authenticated, but not delivered, by the Pass Through Trustee
       then in office, any successor by merger, conversion or
       consolidation to such authenticating Pass Through Trustee may
       adopt such authentication and deliver the Certificates so
       authenticated with the same effect as if such successor Pass
       Through Trustee had itself authenticated such Certificates.

            SECTION 7.12.  Maintenance of Agencies.

            (a)  There shall at all times be maintained in the Borough
       of Manhattan, The City of New York, an office or agency where
       Certificates may be presented or surrendered for registration of
       transfer or for exchange, and for payment thereof and where
       notices and demands to or upon the Pass Through Trustee in
       respect of the Certificates or of this Pass Through Trust
       Agreement may be served.  Such office or agency shall be
       initially at ________, [address].  Written notice of the location
       of each such other office or agency and of any change of location
       thereof shall be given by the Pass Through Trustee to Union, the
       Owner Trustee, the Indenture Trustee, the Equipment Trust
       Trustees and the Certificateholders. In the event that no such
       office or agency shall be maintained or no such notice of
       location or of change of location shall be given, presentations
       and demands may be made and notices may be served at the
       Corporate Trust Office of the Pass Through Trustee.

            (b)  There shall at all times be a Registrar and a Paying
       Agent hereunder.  Each such Authorized Agent shall be a bank or
       trust company, shall be a corporation organized and doing
       business under the laws of the United States of America or any
       State or Territory thereof or of the District of Columbia, with a
       combined capital and surplus of at least $75,000,000, or, if the
       Pass Through Trustee shall be acting as the Registrar or Paying
       Agent hereunder, a corporation the obligations of which are
       guaranteed by a corporation organized and doing business under
       the laws of the United States of America or any State or
       Territory thereof or of the District of Columbia, with a combined
       capital and surplus of at least $75,000,000, and shall be
       authorized under such laws to exercise corporate trust powers,
       subject to supervision by Federal or state authorities.  The Pass
       Through Trustee shall initially be the Paying Agent and, as
       provided in Section 3.4, Registrar hereunder.  Each Registrar
       shall furnish to the Pass Through Trustee, at stated intervals of
       not more than six months, and at such other times as the Pass
       Through Trustee may request in writing, a copy of the Register.

            (c)  Any corporation into which any Authorized Agent may be
       merged or converted or with which it may be consolidated, or any
       corporation resulting from any merger, consolidation or
       conversion to which any Authorized Agent shall be a party, or any
       corporation succeeding to the corporate trust business of any
       Authorized Agent, shall be the successor of such Authorized Agent
       hereunder, if such successor corporation is otherwise eligible
       under this Section, without


                                     -44-

   50


       the execution or filing of any paper or any further act on the
       part of the parties hereto or such Authorized Agent or such
       successor corporation.

            (d)  Any Authorized Agent may at any time resign by giving
       written notice of resignation to the Pass Through Trustee, the
       Companies, the Owner Trustee, the Equipment Trust Trustees and
       the Indenture Trustee.  The Companies may, and at the request of
       the Pass Through Trustee shall, at any time terminate the agency
       of any Authorized Agent by giving written notice of termination
       to such Authorized Agent and to the Pass Through Trustee.  Upon
       the resignation or termination of an Authorized Agent or in case
       at any time any such Authorized Agent shall cease to be eligible
       under this Section (when, in either case, no other Authorized
       Agent performing the functions of such Authorized Agent shall
       have been appointed), the Companies shall promptly appoint one or
       more qualified successor Authorized Agents, reasonably
       satisfactory to the Pass Through Trustee, to perform the
       functions of the Authorized Agent which has resigned or whose
       agency has been terminated or who shall have ceased to be
       eligible under this Section.  Union shall give written notice of
       any such appointment made by them to the Pass Through Trustee,
       the Owner Trustee, the Equipment Trust Trustees and the Indenture
       Trustee; and in each case the Pass Through Trustee shall mail
       notice of such appointment to all Certificateholders as their
       names and addresses appear on the Register.

            (e)  The Companies agree, severally and not jointly, to pay,
       or cause to be paid, pursuant to separate agreements from time to
       time to each Authorized Agent reasonable compensation for its
       services and to reimburse it for its reasonable expenses.

            SECTION 7.13.  Money for Certificate Payments to Be Held in
       Pass Through Trust.  All moneys deposited with any Paying Agent
       for the purpose of any payment on Certificates shall be deposited
       and held in trust for the benefit of the Certificateholders
       entitled to such payment, subject to the provisions of this
       Section.  Moneys so deposited and held in trust shall constitute
       a separate trust fund for the benefit of the Certificateholders
       with respect to which such money was deposited.

            The Pass Through Trustee will cause each Paying Agent other
       than the Pass Through Trustee to execute and deliver to it an
       instrument in which such Paying Agent shall agree with the Pass
       Through Trustee, subject to the provisions of this Section, that
       such Paying Agent will:

                 (i)  hold all sums held by it for payments on
            Certificates in trust for the benefit of the Persons
            entitled thereto until such sums shall be paid to such
            Persons or otherwise disposed of as herein provided;

                 (ii)  give the Pass Through Trustee notice of any
            default by any obligor upon the Certificates in the making
            of any such payment; and


                                     -45-

   51



                 (iii)  at any time during the continuance of any such
            default, upon the written request of the Pass Through
            Trustee, forthwith pay to the Pass Through Trustee all sums
            so held in trust by such Paying Agent.

            The Pass Through Trustee may at any time, for the purpose of
       obtaining the satisfaction and discharge of this Pass Through
       Trust Agreement or for any other purpose, direct any Paying Agent
       to pay to the Pass Through Trustee all sums held in trust by such
       Paying Agent, such sums to be held by the Pass Through Trustee
       upon the same trusts as those upon which such sums were held by
       such Paying Agent; and, upon such payment by any Paying Agent to
       the Pass Through Trustee, such Paying Agent shall be released
       from all further liability with respect to such money.

            SECTION 7.14.  Registration of Equipment Notes and ETCs in
       Pass Through Trustee's Name.  The Pass Through Trustee agrees
       that all Equipment Notes, ETCs, Permitted Investments, if any,
       and Specified Investments, if any, shall be issued in the name of
       the Pass Through Trustee or its nominee and held by the Pass
       Through Trustee, or, if not so held, the Pass Through Trustee or
       its nominee shall be reflected as the owner of such Equipment
       Notes, ETCs, Permitted Investments, or Specified Investments, as
       the case may be, in the register of the issuer of such Equipment
       Notes, ETCs, Permitted Investments or Specified Investments.  In
       no event shall the Pass Through Trustee invest in, or hold,
       Equipment Notes, ETCs, Permitted Investments or Specified
       Investments in a manner that would cause the Pass Through Trustee
       not to have the ownership interest in such Equipment Notes, ETCs,
       Permitted Investments or Specified Investments under the
       applicable provisions of the Uniform Commercial Code in effect
       where the Pass Through Trustee holds such Equipment Notes, ETCs,
       Permitted Investments or Specified Investments or other
       applicable law then in effect.

            SECTION 7.15.  Representations and Warranties of Pass
       Through Trustee.  The Pass Through Trustee hereby represents and
       warrants that:

                 (i)  the Pass Through Trustee is a national banking
            association, validly existing and holding a valid
            certificate to conduct business as a national banking
            association with trust powers under the laws of the United
            States of America;

                 (ii)  the Pass Through Trustee has full power,
            authority and legal right to execute, deliver, and perform
            this Pass Through Trust Agreement and the Participation
            Agreement and has taken all necessary action to authorize
            the execution, delivery, and performance by it of this Pass
            Through Trust Agreement and the Participation Agreement;

                 (iii)  the execution, delivery and performance by the
            Pass Through Trustee of this Pass Through Trust Agreement
            and the Participation Agreement (a) will not violate any
            provision of any United States or Illinois law or regulation
            governing the banking and trust powers of the Pass Through
            Trustee or any order, writ, judgment, or decree of any
            court, arbitrator, or governmental authority applicable to
            the Pass Through Trustee or any of its


                                     -46-

   52


            assets, (b) will not violate any provision of the articles
            of association or by-laws of the Pass Through Trustee, or
            (c) will not violate any provision of, or constitute, with
            or without notice or lapse of time, a default under, or
            result in the creation or imposition of any lien on any
            properties included in the Trust Property pursuant to the
            provisions of any mortgage, indenture, contract, agreement
            or other undertaking to which it is a party, which
            violation, default or lien could reasonably be expected to
            have an adverse effect on the Pass Through Trustee's
            performance or ability to perform its duties hereunder or
            thereunder or on the transactions contemplated herein or
            therein;

                 (iv)  the execution, delivery and performance by the
            Pass Through Trustee of this Pass Through Trust Agreement
            and the Participation Agreement will not require the
            authorization, consent, or approval of, the giving of notice
            to, the filing or registration with, or the taking of any
            other action in respect of, any United States or Illinois
            governmental authority or agency regulating the banking and
            corporate trust activities of the Pass Through Trustee,
            other than any such authorization, consent or approval as
            has been duly obtained or given and is in full force and
            effect; and

                 (v)  this Pass Through Trust Agreement and the
            Participation Agreement have been duly executed and
            delivered by the Pass Through Trustee and constitute the
            legal, valid, and binding agreements of the Pass Through
            Trustee, enforceable against it in accordance with their
            respective terms, provided that enforceability may be
            limited by (x) applicable bankruptcy, insolvency,
            reorganization, moratorium or similar laws affecting the
            rights of creditors generally and (y) general principles of
            equity.

            SECTION 7.16.  Withholding Taxes; Information Reporting.
       The Pass Through Trustee, as trustee of a grantor trust, shall
       exclude and withhold from each distribution of principal,
       Make-Whole Amount, if any, and interest and other amounts due
       hereunder or under the Certificates any and all withholding taxes
       applicable thereto as required by law.  The Pass Through Trustee
       agrees to act as such withholding agent and, in connection
       therewith, whenever any present or future taxes or similar
       charges are required to be withheld with respect to any amounts
       payable in respect of the Certificates, to withhold such amounts
       and timely pay the same to the appropriate authority in the name
       of and on behalf of the Certificateholders, that it will file any
       necessary withholding tax returns or statements when due, and
       that, as promptly as possible after the payment thereof, it will
       deliver to each Certificateholder appropriate documentation
       showing the payment thereof, together with such additional
       documentary evidence as such Certificateholders may reasonably
       request from time to time.  The Pass Through Trustee agrees to
       file any other information reports as it may be required to file
       under United States law.  To the extent that the Pass Through
       Trustee fails, with respect to any Certificateholder, to withhold
       and pay over any such taxes to the appropriate taxing authority,
       the Pass Through Trustee shall, upon a claim being made for such
       taxes by such authority, and before making any claim to Union for
       indemnification under Section 7.1 of the Participation Agreement
       (if such indemnification would otherwise be permissible
       thereunder), take all reasonable steps to recover such taxes from
       such

                                     -47-
 

   53

       Certificateholder, including, without limitation, withholding the
       amount of such taxes from subsequent distributions, if any, to
       such Certificateholder.  To the extent that the Pass Through
       Trustee receives any amount from Union for indemnification of
       such taxes which the Pass Through Trustee thereafter recovers
       from the appropriate Certificateholder (including by withholding
       from subsequent distributions to such Certificateholder), the
       Pass Through Trustee shall reimburse Union therefor.  The Pass
       Through Trustee shall be permitted to rely upon any certificate
       presented by a Certificateholder claiming an exemption from
       withholding absent bad faith on the part of the Pass Through
       Trustee.

            SECTION 7.17.  Trustee's Liens.  The Pass Through Trustee in
       its individual capacity agrees that it will at its own cost and
       expense promptly take such actions as may be necessary to duly
       discharge and satisfy in full all Trustee's Liens.

            SECTION 7.18.  Preferential Collection of Claims.  The Pass
       Through Trustee shall comply with Sections 311(a) and (b) of the
       Trust Indenture Act, excluding any creditor relationship arising
       under Section 311(b) of the Trust Indenture Act.  A Pass Through
       Trustee who has resigned or been removed shall be subject to
       Section 311(a) of the Trust Indenture Act to the extent
       indicated.


                                  ARTICLE VIII

         CERTIFICATEHOLDERS' LISTS AND REPORTS BY PASS THROUGH TRUSTEE

            SECTION 8.1.  The Companies to Furnish Pass Through Trustee
       with Names and Addresses of Certificateholders.  The Companies
       will furnish to the Pass Through Trustee within 15 days after
       each Record Date with respect to a Scheduled Payment, and at such
       other times as the Pass Through Trustee may request in writing,
       within 30 days after receipt by the Companies of any such
       request, a list, in such form as the Pass Through Trustee may
       reasonably require, of all information in the possession or
       control of the Companies as to the names and addresses of the
       Certificateholders, in each case as of a date not more than 15
       days prior to the time such list is furnished; provided, however,
       that so long as the Pass Through Trustee is the sole Registrar,
       no such list need be furnished; and provided further,  however,
       that no such list need be furnished for so long as a copy of the
       Register is being furnished to the Pass Through Trustee pursuant
       to Section 7.12.

            SECTION 8.2.  Preservation of Information; Communications to
       Certificateholders.  The Pass Through Trustee shall preserve, in
       as current a form as is reasonably practicable, the names and
       addresses of Certificateholders contained in the most recent
       Register or list furnished to the Pass Through Trustee as
       provided in Section 7.12 or Section 8.1, as the case may be, and
       the names and addresses of Certificateholders received by the
       Pass Through Trustee in its capacity as Registrar, if so acting.
       The Pass Through Trustee may destroy any Register or list
       furnished


                                     -48-

   54


       to it as provided in Section 7.12 or Section 8.1, as the case may
       be, upon receipt of a new Register or list so furnished.

            SECTION 8.3.  Reports by Pass Through Trustee.

            (a)  Within 60 days after ________ of each year commencing
       with the year ______, the Pass Through Trustee shall transmit to
       the Certificateholders, as provided in Section 313(c) of the
       Trust Indenture Act, a report dated as of such __________, if
       required by Section 313(a) of the Trust Indenture Act.  The Pass
       Through Trustee shall also comply with Section 313(b) of the
       Trust Indenture Act.

            (b)  A copy of each such report shall, at the time of
       transmission to Certificateholders, be filed by the Pass Through
       Trustee with each stock exchange (if any) upon which the
       Certificates are listed, and also with the Commission.  The
       Companies will notify the Pass Through Trustee when the
       Certificates are listed on any stock exchange and any subsequent
       change with respect thereto.

            SECTION 8.4.  Reports by the Company.  Union shall [and, if
       Procor becomes subject to the periodic reporting requirements of
       Section 13 or 15(d) of the Exchange Act, Procor shall]:

            (a)  file with the Pass Through Trustee, within 30 days
       after it is required to file the same with the Commission, copies
       of the annual reports and of the information, documents and other
       reports (or copies of such portions of any of the foregoing as
       the Commission may from time to time by rules and regulations
       prescribe) which it is required to file with the Commission
       pursuant to Section 13 or Section 15(d) of the Securities
       Exchange Act of 1934, as amended; or if it is not required to
       file information, documents or reports pursuant to either of such
       sections, then to file with the Pass Through Trustee and the
       Commission, in accordance with rules and regulations prescribed
       by the Commission pursuant to Section 314(a)(1) of the Trust
       Indenture Act, such of the supplementary and periodic
       information, documents and reports which may be required pursuant
       to Section 13 of the Securities Exchange Act of 1934, as amended,
       in respect of a security listed and registered on a national
       securities exchange as may be prescribed in such rules and
       regulations;

            (b)  file with the Pass Through Trustee and the Commission,
       in accordance with the rules and regulations prescribed by the
       Commission, such additional information, documents and reports
       with respect to compliance by it with the conditions and
       covenants provided for in this Agreement, as may be required by
       such rules and regulations, including, in the case of annual
       reports, if required by such rules and regulations, certificates
       or opinions of independent public accountants, conforming to the
       requirements of Section 1.2;

            (c)  transmit to all Certificateholders, in the manner and
       to the extent provided in Section 313(c) of the Trust Indenture
       Act such summaries of any information, documents and

                                     -49-
 

   55

       reports required to be filed by Union [and Procor] pursuant to
       subsections (a) and (b) of this Section 8.4 as may be required by
       rules and regulations prescribed by the Commission; and

            (d)  furnish to the Pass Through Trustee, not less often
       than annually, a certificate from the principal executive
       officer, principal financial officer or principal accounting
       officer thereof as to his or her knowledge of Union's [and
       Procor's] compliance with all conditions and covenants under this
       Agreement.  For purposes of this subsection (d), such compliance
       shall be determined without regard to any period of grace or
       requirement of notice provided under this Agreement.  [Until such
       time as Procor becomes a reporting company under the Exchange
       Act, such annual certificate may be furnished as to both Union
       and Procor by a Responsible Officer of Union.]


                                   ARTICLE IX

                  SUPPLEMENTS TO PASS THROUGH TRUST AGREEMENT

            SECTION 9.1.  Supplements to Pass Through Trust Agreement
       Without Consent of Certificateholders.  Without the consent of
       the Certificateholders, the Companies may, and the Pass Through
       Trustee (subject to Section 9.3) shall, at any time and from time
       to time enter into one or more agreements supplemental hereto, in
       form satisfactory to the Pass Through Trustee, for any of the
       following purposes:

                 (i)  to evidence the succession of another corporation
            to Union [or Procor] and the assumption by any such
            successor of the covenants of Union [or Procor] herein
            contained;

                 (ii)  to add to the covenants of either of the
            Companies for the benefit of the Certificateholders, or to
            surrender any right or power herein conferred upon the
            Companies;

                 (iii)  to cure any ambiguity, to correct any manifest
            error to correct or supplement any provision herein which
            may be defective or inconsistent with any other provision
            herein or to make any other provisions with respect to
            matters or questions arising under this Pass Through Trust
            Agreement, provided that any such action shall not adversely
            affect the interests of the Certificateholders; or

                 (iv)  to modify, eliminate or add to the provisions of
            this Pass Through Trust Agreement to such extent as shall be
            necessary to continue the qualification of this Pass Through
            Trust Agreement (including any supplemental agreement) under
            the Trust Indenture Act, or under any similar Federal
            statute hereafter enacted, and to add to this Pass Through
            Trust Agreement such other provisions as may be expressly
            permitted by the Trust Indenture Act, excluding, however,
            the provisions referred to in Section 316(a)(2) of the Trust
            Indenture Act as in effect at the date as of which this


                                     -50-

   56


            instrument was executed or any corresponding provision in
            any similar Federal statute hereafter enacted.

            SECTION 9.2.  Supplements to Pass Through Trust Agreement
       with Consent of Certificateholders.  With the consent of the
       Certificateholders holding Certificates evidencing Fractional
       Undivided Interests aggregating not less than a majority in
       interest, by Act of such Certificateholders delivered to the
       Companies and the Pass Through Trustee, the Companies may (with
       the consent of the Owner Trustee, if any, such consent not to be
       unreasonably withheld), and the Pass Through Trustee (subject to
       Section 9.3) shall, enter into an agreement or agreements
       supplemental hereto for the purpose of adding any provisions to
       or changing in any manner or eliminating any of the provisions of
       this Pass Through Trust Agreement or of modifying in any manner
       the rights and obligations of the Certificateholders under this
       Pass Through Trust Agreement; provided, however, that no such
       supplemental agreement shall, without the consent of each
       Certificateholder of an Outstanding Certificate affected thereby:

                 (i)  reduce in any manner the amount of, or delay the
            timing of, any receipt by the Pass Through Trustee of
            payments on the Equipment Notes or the ETCs held in the Pass
            Through Trust or distributions that are required to be made
            herein on any Certificate, or change any date of payment on
            any Certificate, or change the place of payment where, or
            the coin or currency in which, any Certificate is payable,
            or impair the right to institute suit for the enforcement of
            any such payment or distribution on or after the Regular
            Distribution Date or Special Distribution Date applicable
            thereto;

                 (ii)  permit the disposition of any Equipment Note or
            ETCs in the Trust Property except as permitted by this Pass
            Through Trust Agreement, or otherwise deprive any
            Certificateholder of the beneficial ownership of the
            Equipment Notes or ETCs in the Pass Through Trust;

                 (iii)  reduce the percentage of the aggregate
            Fractional Undivided Interests of the Pass Through Trust
            required for any such supplemental agreement, or reduce such
            percentage required for any waiver (of compliance with
            certain provisions of this Pass Through Trust Agreement or
            certain defaults hereunder and their consequences) provided
            or in this Pass Through Trust Agreement;

                 (iv)  modify Article XII or otherwise release Union
            from its obligations under the Guarantee; or

                 (v)  modify any of the provisions of this Section or
            Section 6.5, except to increase any such percentage or to
            provide that certain other provisions of this Pass Through
            Trust Agreement cannot be modified or waived without the
            consent of the Certificateholder of each Certificate
            affected thereby.


                                     -51-

   57



            It shall not be necessary for any Act of Certificateholders
       under this Section to approve the particular form of any proposed
       supplemental agreement, but it shall be sufficient if such Act
       shall approve the substance thereof.

            SECTION 9.3.  Documents Affecting Immunity or Indemnity.  If
       in the opinion of the Pass Through Trustee any document required
       to be executed by it pursuant to the terms of Section 9.1 or 9.2
       affects any interest, right, duty, obligation, immunity or
       indemnity in favor of the Pass Through Trustee under this Pass
       Through Trust Agreement, the Pass Through Trustee may in its
       discretion decline to execute such document.

            SECTION 9.4.  Execution of Supplements to Pass Through Trust
       Agreements.  In executing, or accepting the additional trusts
       created by, any supplemental agreement permitted by this Article
       or the modifications thereby of the trusts created by this Pass
       Through Trust Agreement, the Pass Through Trustee shall be
       entitled to receive, and shall be fully protected in relying
       upon, an Opinion of Counsel stating that the execution of such
       supplemental agreement is authorized or permitted by this Pass
       Through Trust Agreement.

            SECTION 9.5.  Effect of Supplements to Pass Through Trust
       Agreement.  Upon the execution of any supplemental agreement
       under this Article, this Pass Through Trust Agreement shall be
       modified in accordance therewith, and such supplemental agreement
       shall form a part of this Pass Through Trust Agreement for all
       purposes; and every Certificateholder of Certificates theretofore
       or thereafter authenticated and delivered hereunder shall be
       bound thereby.

            SECTION 9.6.  Conformity with Trust Indenture Act.  Every
       supplemental agreement executed pursuant to this Article shall
       conform to the requirements of the Trust Indenture Act as then in
       effect.

            SECTION 9.7.  Reference in Certificates to Supplements to
       Pass Through Trust Agreements.  Certificates authenticated and
       delivered after the execution of any supplemental agreement
       pursuant to this Article may bear a notation in form approved by
       the Pass Through Trustee as to any matter provided for in such
       supplemental agreement, and, in such case, suitable notation may
       be made upon Outstanding Certificates after proper presentation
       and demand.


                                   ARTICLE X

                    AMENDMENTS TO INDENTURE, NOTE DOCUMENTS,
                  EQUIPMENT TRUST AGREEMENTS AND ETC DOCUMENTS

            In the event that the Pass Through Trustee, as holder of any
       Equipment Note or an ETC in trust for the benefit of the
       Certificateholders, receives a request for a consent to any
       amendment, modification, waiver or supplement under the
       Indenture, other Note Document, either


 
                                     -52-
   58


       Equipment Trust Agreement, other ETC Document or the
       Participation Agreement, the Pass Through Trustee shall forthwith
       send a notice of such proposed amendment modification, waiver or
       supplement, to each Certificateholder registered on the Register
       as of such date.  The Pass Through Trustee shall request from the
       Certificateholders Directions as to (a) whether or not to direct
       the Indenture Trustee or such Equipment Trust Trustee, as the
       case may be, to take or refrain from taking any action which a
       holder of such Equipment Note or ETC, as the case may be, has the
       option to direct, (b) whether or not to give or execute any
       waivers, consents, amendments, modifications or supplements as a
       holder of such Equipment Note or ETC, as the case may be, and (c)
       how to vote any Equipment Note or ETC, as the case may be, if a
       vote has been called for with respect thereto.  Provided such a
       request for Certificateholder Direction shall have been made, in
       directing any action or casting any vote or giving any consent as
       the holder of any Equipment Note or ETC, as the case may be, the
       Pass Through Trustee shall vote or consent with respect to such
       Equipment Note or ETC, as the case may be, in the same proportion
       as the Certificates were actually voted by Acts of
       Certificateholders delivered to the Pass Through Trustee prior to
       two Business Days before the Pass Through Trustee directs such
       action or casts such vote or gives such consent.  Notwithstanding
       the foregoing, but subject to Section 6.4, in the case that an
       Event of Default hereunder shall have occurred and be continuing,
       the Pass Through Trustee may, in its own discretion and at its
       own direction, consent and notify the Indenture Trustee or
       Equipment Trust Trustee of such consent to any amendment,
       modification, waiver or supplement under the Indenture, and other
       Note Document, any Equipment Trust Agreement, any other ETC
       Document or the Participation Agreement.


                                   ARTICLE XI

                       TERMINATION OF PASS THROUGH TRUST

            The respective obligations and responsibilities of the
       Companies and the Pass Through Trustee created hereby and the
       Pass Through Trust created hereby shall terminate upon the
       distribution to all Certificateholders and the Pass Through
       Trustee of all amounts required to be distributed to them
       pursuant to this Pass Through Trust Agreement and the disposition
       of all property held as part of the Trust Property; provided,
       however, that in no event shall the trust created hereby continue
       beyond the expiration of 21 years from the death of the last
       survivor of the descendants of John D. Rockefeller, father of
       Nelson Rockefeller, former Vice President of the United States,
       living on the date of this Pass Through Trust Agreement.

            Notice of any termination, specifying the Regular
       Distribution Date (or Special Distribution Date, as the case may
       be) upon which the Certificateholders may surrender their
       Certificates to the Pass Through Trustee for payment of the final
       distribution and cancellation, shall be mailed promptly by the
       Pass Through Trustee to Certificateholders not earlier than the
       60th day and not later than the 20th day next preceding such
       final distribution, specifying (a) the Regular Distribution Date
       (or Special Distribution Date, as the case may be) upon which the

                                     -53-
 

   59


       proposed final payment of the Certificates will be made upon
       presentation and surrender of Certificates at the office or
       agency of the Pass Through Trustee therein specified, (b) the
       amount of any such proposed final payment, and (c) that the
       Record Date otherwise applicable to such Regular Distribution
       Date (or Special Distribution Date, as the case may be) is not
       applicable, payments being made only upon presentation and
       surrender of the Certificates at the office or agency of the Pass
       Through Trustee therein specified.  The Pass Through Trustee
       shall give such notice to the Registrar at the time such notice
       is given to Certificateholders. Upon presentation and surrender
       of the Certificates, the Pass Through Trustee shall cause to be
       distributed to Certificateholders amounts distributable on such
       Regular Distribution Date (or Special Distribution Date, as the
       case may be) pursuant to Section 4.2.

            In the event that all of the Certificateholders shall not
       surrender their Certificates for cancellation within six months
       after the date specified in the above-mentioned written notice,
       the Pass Through Trustee shall give a second written notice to
       the remaining Certificateholders to surrender their Certificates
       for cancellation and receive the final distribution with respect
       thereto.  In the event that any money held by the Pass Through
       Trustee for the payment of distributions on the Certificates
       shall remain unclaimed for two years and 11 months after the
       final distribution date with respect thereto, the Pass Through
       Trustee shall pay to the Indenture Trustee or each Equipment
       Trust Trustee the appropriate amount of money relating to the
       Indenture Trustee or such Equipment Trust Trustee and shall give
       written notice thereof to the Owner Trustee and Union.


                                  ARTICLE XII

                               GUARANTEE OF UNION

            [SECTION 12.1.  Guarantee.  Subject to the provisions of
       this Article XII, Union hereby irrevocably and unconditionally
       guarantees, as a primary obligor and not a surety, to each
       Certificateholder of a Certificate now or hereafter authenticated
       and delivered by the Pass Through Trustee and to the Pass Through
       Trustee and its successors and assigns, irrespective of the
       validity and enforceability of this Pass Through Trust Agreement,
       the Certificates hereunder, the Procor Equipment Trust Agreement
       or the Procor ETC thereunder, the due and punctual distribution
       to Certificateholders of the amounts of principal of, and
       interest (including post-petition interest in any proceeding
       under any applicable bankruptcy law whether or not permitted
       thereby), and interest on overdue principal and interest, if any,
       to the extent lawful, payable with respect to the Procor ETC when
       and as the same shall become due and payable, whether by
       acceleration thereof, or otherwise (including amounts that would
       become due but for the operation of the automatic stay under
       applicable bankruptcy law), in accordance with the terms of the
       Procor ETC and of this Pass Through Trust Agreement.  Union also
       hereby guarantees for the benefit of the Persons identified in
       the preceding sentence, the due and punctual performance by
       Procor of its obligations under the Procor Equipment Trust
       Agreement.   Union hereby agrees that its obligations

                                     -54-
 

   60


       hereunder shall be absolute and unconditional, irrespective of,
       and shall be unaffected by, any failure to enforce the provisions
       of the Procor ETC or this Pass Through Trust Agreement, any
       waiver, modification or indulgence granted to Procor with respect
       thereto by the Certificateholders, the Equipment Trust Trustee or
       the Pass Through Trustee, or any other circumstances which may
       otherwise constitute a legal or equitable discharge of a surety
       or guarantor.  Union hereby waives diligence, presentment, filing
       of claims with a court in the event of a merger or bankruptcy of
       Procor, any right to require a  proceeding first against Procor,
       the benefit of discussion, protest or notice with respect to the
       Procor ETC or the indebtedness evidenced thereby and all demands
       whatsoever, and covenants that this Guarantee shall not be
       discharged as to the Procor ETC except by payment in full of the
       principal thereof and all accrued interest thereon and all other
       amounts due with respect thereto.

            Union shall be subrogated to all rights of the
       Certificateholders against Procor in respect of any amounts paid
       to the Certificateholders by Union pursuant to the provisions of
       this Guarantee; provided, however, that Union shall not be
       entitled to enforce, or to receive any payments arising out of or
       based upon, such right of subrogation until all amounts payable
       to the Pass Through Trustee hereunder with respect to the Procor
       ETC shall have been paid in full.

            The Guarantee set forth in this Section 12.1 shall not be
       valid or become obligatory for any purpose with respect to a
       Certificate until the certificate of authentication on such
       Certificate shall have been signed by or on behalf of the Pass
       Through Trustee.

            The parties hereto acknowledge that payments by Union
       hereunder with respect to (i) principal attributable to the
       Procor ETC shall have the effect of reducing on a dollar for
       dollar basis the outstanding principal amount of the Procor ETC
       and (ii) accrued interest attributable to the Procor ETC shall
       have the effect of reducing on a dollar for dollar basis the
       outstanding accrued interest on the Procor ETC.  For the purposes
       of this paragraph only, payments by Union hereunder shall first
       be applied against outstanding accrued interest then against
       outstanding principal.

            SECTION 12.2.  Execution and Delivery of Guarantee.

            (a) To evidence the Guarantee set forth in this Article XII,
       Union hereby agrees that a notation of the Guarantee shall be
       placed on each Certificate authenticated and delivered by the
       Pass Through Trustee on or after the date the Trust Property
       includes the Procor ETC.
            (b) A Responsible Officer of Union shall sign the notation
       of Guarantee on the Certificates by manual or facsimile
       signature. If a Responsible Officer whose signature is on the
       notation of Guarantee no longer holds that office at the time the
       Pass Through Trustee authenticates the Certificate on which the
       Guarantee is endorsed, the Guarantee shall be valid nevertheless.
       Union hereby agrees that the Guarantee set forth in Section 12.1
       shall remain in full

                                     -55-
 

   61


       force and effect notwithstanding any failure to endorse on each
       Certificate a notation of the Guarantee.

            (c) The delivery of any Certificate by the Pass Through
       Trustee, after the authentication thereof hereunder, shall
       constitute due delivery of the Guarantee set forth in this Pass
       Through Trust Agreement on behalf of Union.

            SECTION 12.3.  Limitation of Union's Liability.  Union
       hereby confirms that it is its intention that the Guarantee not
       constitute a fraudulent transfer or conveyance for purposes of
       any bankruptcy law, fraudulent conveyance act, or any similar
       law.  To effectuate the foregoing intention, Union hereby
       irrevocably agrees that its obligations under the Guarantee shall
       be limited to the maximum amount as will, after giving effect to
       such maximum amount and all other (contingent or otherwise)
       liabilities of Union that are relevant under such laws, and after
       giving effect to any rights to contribution pursuant to any
       agreement providing for an equitable contribution among Union and
       other Affiliates of Procor, such maximum amount shall result in
       the obligations of Union in respect of such maximum amount not
       constituting a fraudulent transfer or conveyance.

            SECTION 12.4.  Guarantee Unconditional.  Upon failure of
       payment when due of any amount so guaranteed for whatever reason,
       Union will be obligated to pay the same immediately.  Union
       hereby agrees that its obligations hereunder shall be continuing,
       absolute and unconditional, irrespective of:  any delays in
       obtaining or realizing upon or failure to obtain or realize upon
       Trust Property; the recovery of any judgment against Procor or
       Union; any extension, renewal settlement, compromise, waiver or
       release in respect of any obligation of Procor under this Pass
       Through Trust Agreement or the Procor ETC, by operation of law or
       otherwise; any modification or amendment of or supplement to this
       Pass Through Trust Agreement or the Procor ETC; any change in the
       corporate existence, structure or ownership of Procor, or any
       insolvency, bankruptcy, reorganization or other similar
       proceeding affecting Procor or its assets or any resulting
       release or discharge of any obligation of Procor contained in
       this Pass Through Trust Agreement or the Procor ETC; the
       existence of any claim, set-off or other rights which Union may
       have at any time against Procor, the Pass Through Trustee, any
       Certificateholder or any other Person, whether in connection
       herewith or any unrelated transactions; provided, that nothing
       herein shall prevent the assertion of any such claim by separate
       suit or compulsory counterclaim; any invalidity or
       unenforceability relating to or against Procor for any reason of
       this Pass Through Trust Agreement or the Procor ETC, or any
       provision of applicable law or regulation purporting to prohibit
       the payment by Procor of the principal of or interest on the
       Procor ETC or any other amount payable by Procor under this Pass
       Through Trust Agreement, or the Procor ETC; or any other act or
       omission to act or delay of any kind by Procor, the Pass Through
       Trustee, any Certificateholder, or any other Person or any other
       circumstance whatsoever which might, but for the provisions of
       this paragraph, constitute a legal or equitable discharge of
       Union's obligations hereunder.  Union hereby waives diligence,
       presentment, demand of payment, filing of claims with a court in
       the event of insolvency or


                                     -56-

   62


       bankruptcy of Procor, any right to require a proceeding first
       against Procor, protest, notice and all demand whatsoever and
       covenants that this Guarantee will not be discharged except by
       the complete performance of the obligations contained in the
       Procor ETC, this Pass Through Trust Agreement and in this Article
       XII.  Union's obligations hereunder shall remain in full force
       and effect until this Pass Through Trust Agreement shall have
       terminated and the principal of and interest on the Procor ETC
       and all other obligations payable by Procor relating thereto
       shall have been paid in full.  If at any time the distribution of
       any payment of the principal of or interest with respect to the
       Procor ETC or any other amount payable by Procor under this Pass
       Through Trust Agreement is rescinded or must be otherwise
       restored or returned upon the insolvency, bankruptcy or
       reorganization of Procor or otherwise, Union's obligations
       hereunder with respect to such payment shall be reinstated as
       though such payment had been due but not made at such time, and
       this Article XII, to the extent theretofore discharged, shall be
       reinstated in full force and effect.  Union irrevocably waives
       any and all rights to which it may be entitled, by operation of
       law or otherwise, upon making any payment hereunder to be
       subrogated to the rights of the payee against Procor with respect
       to such payment or otherwise to be reimbursed, indemnified or
       exonerated by Procor in respect thereof until all obligations of
       Procor under the Procor ETC are satisfied.  Union further agrees
       that, as between Union, on the one hand, and the
       Certificateholders and the Pass Through Trustee, on the other
       hand, (i) the maturity of the obligations guaranteed hereby may
       be accelerated as provided in Article VI hereof for the purposes
       of this Guarantee, notwithstanding any stay, injunction or other
       prohibition preventing such acceleration in respect of the
       obligations guaranteed hereby and (ii) in the event of any
       declaration of acceleration of such obligations as provided in
       Article VI hereof, such obligations (whether or not due and
       payable) shall forthwith become due and payable by Union for the
       purpose of this Article XII.


                                  ARTICLE XIII

                            MISCELLANEOUS PROVISIONS

            SECTION 13.1.  Limitation on Rights of Certificateholders.
       The death or incapacity of any Certificateholder shall not
       operate to terminate this Pass Through Trust Agreement or the
       Pass Through Trust, nor entitle such Certificateholder's legal
       representatives or heirs to claim an accounting or to take any
       action or commence any proceeding in any court for a partition or
       winding up of the Pass Through Trust, nor otherwise affect the
       rights, obligations and liabilities of the parties hereto or any
       of them.

            SECTION 13.2.  Certificates Nonassessable and Fully Paid.
       Certificateholders shall not be personally liable for obligations
       of the Pass Through Trust, the Fractional Undivided Interests
       represented by the Certificates shall be nonassessable for any
       losses or expenses of the Pass Through Trust or for any reason
       whatsoever, and Certificates upon authentication thereof by the
       Pass Through Trustee pursuant to Section 3.2 are and shall be
       deemed fully paid.  No


                                     -57-

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       Certificateholder shall have any right (except as expressly
       provided herein) to vote or in any manner otherwise control the
       operation and management of the Trust Property, the Pass Through
       Trust established hereunder, or the obligations of the parties
       hereto, nor shall anything set forth herein, or contained in the
       terms of the Certificates, be construed so as to constitute the
       Certificateholders from time to time as partners or members of an
       association.

            SECTION 13.3.  Notices.

            (a) Unless otherwise expressly specified or permitted by the
       terms hereof, all communications and notices provided for herein
       shall be in writing, and any such notice shall become effective
       (i) upon personal delivery thereof, including, without limitation
       by overnight mail and courier service, (ii) in the case of notice
       by United States mail, certified or registered, postage prepaid,
       return receipt requested, three days after being so deposited in
       the United States mail, or (iii) in the case of notice by
       facsimile transmission, upon transmission thereof, provided such
       transmission is promptly confirmed (which conformation may be
       mechanical), in each case addressed to each party here to at the
       addresses set forth below:

            if to Union, to:


                    Union Tank Car Company
                    225 West Washington Street
                    Chicago, Illinois  60606
                    Attention:     Robert W. Webb, Secretary
                    Facsimile:     (312) 845-5305
                    Telephone:     (312) 372-9500


            if to Procor, to:


                    Procor Limited
                    2001 Speers Road
                    Oakville, Ontario
                    Canada L6J 5E1
                    Attention:  Frank Lester, President
                    Facsimile:  (905) 827-0800
                    Telephone:  (905) 827-4111



                                     -58-
 

   64

         with a copy to:

                   Neal, Gerber & Eisenberg
                   2 North LaSalle Street
                   Chicago, Illinois  60602
                   Attention: William M. Holzman, Esq.
                   Facsimile: (312) 269-1747
                   Telephone: (312) 269-8000

         if to the Pass Through Trustee, to:

                   [address]

                   Attention:
                   Facsimile:
                   Telephone:


            (b)  The Companies and the Pass Through Trustee, by notice
       to the other parties, may designate additional or different
       addresses for subsequent notices or communications.

            (c)  Any notice or communication to Certificateholders shall
       be mailed by first-class mail to the addresses for
       Certificateholders shown on the Register kept by the Registrar
       and to addresses filed with the Pass Through Trustee for
       Certificate Owners.  Failure so to mail a notice or communication
       or any defect in such notice or communication shall not affect
       its sufficiency with respect to other Certificateholders.

            (d)  If a notice or communication is mailed in the manner
       provided above within the time prescribed, it is conclusively
       presumed to have been duly given, whether or not the addressee
       receives it.

            (e)  If the Companies mail a notice or communication to the
       Certificateholders, it shall mail a copy to the Pass Through
       Trustee and to each Paying Agent at the same time.

            (f)  Notwithstanding the foregoing, all communications or
       notices to the Pass Through Trustee shall be deemed to be given
       only when received by a Responsible Officer of the Pass Through
       Trustee.

            (g)  The Pass Through Trustee shall promptly furnish the
       Companies with a copy of any demand, notice or written
       communication received by the Pass Through Trustee hereunder from
       any Certificateholder, the Owner Trustee, any Equipment Trust
       Trustee or the Indenture Trustee.

                                     -59-
 

   65



            SECTION 13.4.  Communication by Certificateholder with Other
       Certificateholders.  Certificateholders may communicate pursuant
       to Section 312(b) of the Trust Indenture Act with other
       Certificateholders with respect to their rights under this Pass
       Through Trust Agreement or under the Certificates, and the Pass
       Through Trustee shall comply with Section 312(b) of the Trust
       Indenture Act.  The Companies and the Pass Through Trustee shall
       have the protection of Section 312(c) of the Trust Indenture Act.

            SECTION 13.5.  Governing Law.  THIS PASS THROUGH TRUST
       AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH
       THE LAWS OF THE STATE OF NEW YORK WITHOUT GIVING EFFECT TO ANY
       CHOICE OF LAW OF CONFLICT OF LAW PROVISION OR RULE AND THE
       OBLIGATIONS, RIGHTS, AND REMEDIES OF THE PARTIES HEREUNDER SHALL
       BE DETERMINED IN ACCORDANCE WITH SUCH LAWS.

            SECTION 13.6.  Severability of Provisions.  Whenever
       possible, each provision of this Pass Through Trust Agreement
       shall be interpreted in such manner as to be effective and valid
       under applicable law, but if any provision of this Pass Through
       Trust Agreement shall be prohibited by or invalid under
       applicable law, such provision shall be ineffective to the extent
       of such prohibition or invalidity, without invalidating the
       remainder of such provision or the remaining provisions of this
       Pass Through Trust Agreement.

            SECTION 13.7.  Trust Indenture Act Controls.  This Agreement
       is subject to the provisions of the Trust Indenture Act and
       shall, to the extent applicable, be governed by such provisions.

            SECTION 13.8.  Effect of Headings and Table of Contents. The
       Article and Section headings herein and the Table of Contents are
       for convenience only and shall not affect the construction
       hereof.

            SECTION 13.9.  Successors and Assigns.  All covenants,
       agreements, representations and warranties in this Pass Through
       Trust Agreement by the Pass Through Trustee, the Companies shall
       bind and, to the extent permitted hereby, shall inure to the
       benefit of and be enforceable by their respective successors and
       assigns, whether so expressed or not.

            SECTION 13.10.  Benefits of Pass Through Trust Agreement.
       Nothing in this Pass Through Trust Agreement or in the
       Certificates, express or implied, shall give to any Person, other
       than the parties hereto and their successors hereunder, and the
       Certificateholders, any benefit or any legal or equitable right,
       remedy or claim under this Pass Through Trust Agreement.

            SECTION 13.11.  Legal Holidays.  In any case where any
       Regular Distribution Date or Special Distribution Date relating
       to any Certificate shall not be a Business Day, then
       (notwithstanding any other provision of this Pass Through Trust
       Agreement) payment need not


 
                                     -60-
   66

       be made on such date, but may be made on the next succeeding
       Business Day with the same force and effect as if made on such
       Regular Distribution Date or Special Distribution Date, and no
       interest shall accrue during the intervening period.

            SECTION 13.12.  Counterparts.  For the purpose of
       facilitating the execution of this Pass Through Trust Agreement
       and for other purposes, this Pass Through Trust Agreement may be
       executed simultaneously in any number of counterparts, each of
       which counterparts shall be deemed to be an original, and all of
       which counterparts shall constitute but one and the same
       instrument.


                                     -61-

   67


            IN WITNESS WHEREOF, Union, Procor and the Pass Through
       Trustee have caused this Pass Through Trust Agreement to be duly
       executed by their respective officers, duly attested, all on the
       day and year first above written.

                                   UNION TANK CAR COMPANY




Attest:                            By______________________________
                                     Title:


__________________________
Title:                     

                                   PROCOR LIMITED




Attest:                            By______________________________
                                     Title:


__________________________ 
Title:
                                   By______________________________
                                      Title:
Attest:


__________________________
Title:                    

                                   ___________________,
                                   as Pass Through Trustee



Attest:                            By______________________________
                                     Title:



__________________________
Title:

 

                                     -62-

   68


                                                                       EXHIBIT A

                              FORM OF CERTIFICATE

            Unless this certificate is presented by an authorized
       representative of The Depository Trust Company, a New York
       corporation ("DTC"), to issuers or their agent for registration
       of transfer, exchange or payment, and any Certificate issued is
       registered in the name of Cede & Co. or in such other name as is
       requested by an authorized representative of DTC (and any payment
       is made to Cede & Co. or to such other entity as is requested by
       an authorized representative of DTC), ANY TRANSFER, PLEDGE OR
       OTHER USE HEREOF FOR VALUE OR OTHERWISE BY OR TO ANY PERSON IS
       WRONGFUL inasmuch the registered owner hereof, Cede & Co., has an
       interest herein.


                             UNION TANK CAR COMPANY

                                 PROCOR LIMITED

                           _______ PASS THROUGH TRUST

                           Pass Through Certificate,
                                 Series _______

                   Final Distribution Date:  ________________

              evidencing a fractional undivided interest in a
              trust, the property of which includes certain
              equipment notes (the "Equipment Notes") and
              Equipment Trust Certificates ("ETCs") each secured
              by equipment leased to Union Tank Car Company or
              conditionally sold to Procor Limited.


             Certificate
             No. ___      $__________ Fractional Undivided Interest
                          representing 100% of the Trust


            THIS CERTIFIES THAT Cede & Co., for value received, is the
       registered owner of a $___________
       (____________________________________________________________
       ___________________________________________________ dollars)
       Fractional Undivided Interest in the Union Tank Car Company
       ______ Pass Through Trust (the "Trust") created by
       _______________, a national banking association, as trustee (the
       "Pass Through Trustee"), pursuant to a Pass Through Trust
       Agreement ______ dated _______ (the "Pass Through Trust
       Agreement") among the Pass Through Trustee, Union Tank Car
       Company, a Delaware


 

   69

       corporation ("Union"), and Procor Limited, a Canadian corporation
       and an indirect, wholly-owned subsidiary of Union ("Procor"),
       (Union and Procor are referred to herein collectively as the
       "Companies"), a summary of certain of the pertinent provisions of
       which is set forth below.  To the extent not otherwise defined
       herein, the capitalized terms used herein have the meanings
       assigned to them in the Pass Through Trust Agreement.  This
       Certificate is one of the duly authorized Certificates designated
       as "Pass Through Certificates, Series ______" (herein called the
       "Certificates").  This Certificate is issued under and is subject
       to the terms, provisions, and conditions of the Pass Through
       Trust Agreement, to which Pass Through Trust Agreement the
       Certificateholder of this Certificate by virtue of the acceptance
       hereof assents and by which such Certificateholder is bound.  The
       property of the Pass Through Trust (the "Trust Property")
       includes certain Equipment Notes and ETCs and rights under a
       Union guarantee of the due and punctual distribution to
       Certificateholders of principal and interest payable with respect
       to the Procor ETC.  Each issue of Equipment Notes and ETCs is
       secured by, among other things, a security interest in Equipment
       leased to Union or conditionally sold to Procor, as the case may
       be.

            Subject to and in accordance with the terms of the Pass
       Through Trust Agreement, from funds then available to the Pass
       Through Trustee, there will be distributed on _______ and ______
       of each year (each a "Regular Distribution Date"), commencing on
       _________, to the Person in whose name this Certificate is
       registered at the close of business on the Business Day preceding
       the Regular Distribution Date, an amount in respect of the
       Scheduled Payments on the Equipment Notes and the ETCs due on
       such Regular Distribution Date, the receipt of which has been
       confirmed by the Pass Through Trustee, equal to the product of
       the percentage interest in the Pass Through Trust evidenced by
       this Certificate and an amount equal to the sum of such Scheduled
       Payments.  Subject to and in accordance with the terms of the
       Pass Through Trust Agreement, in the event that Special Payments
       on the Equipment Notes or the ETCs are received by the Pass
       Through Trustee, from funds then available to the Pass Through
       Trustee, there shall be distributed on the applicable Special
       Distribution Date, to the Person in whose name this Certificate
       is registered at the close of business on the Business Day
       preceding the Special Distribution Date, an amount in respect of
       such Special Payments on the Equipment Notes or the ETCs, the
       receipt of which has been confirmed by the Pass Through Trustee,
       equal to the product of the percentage interest in the Pass
       Through Trust evidenced by this Certificate and an amount equal
       to the sum of such Special Payments so received.  If a Regular
       Distribution Date or Special Distribution Date is not a Business
       Day, distribution shall be made on the immediately following
       Business Day with the same force and effect as if made on such
       Regular Distribution Date or Special Distribution Date and no
       interest shall accrue during the intervening period.  The Special
       Distribution Date shall be the second day of the month, or such
       other date, if any, determined as provided in the Pass Through
       Trust Agreement.  The Pass Through Trustee shall mail notice of
       each Special Payment and the Special Distribution Date therefor
       to the Certificateholder of this Certificate.


                                     A-2

   70
            Distributions on this Certificate will be made by the Pass
       Through Trustee by wire transfer of funds to the Person entitled
       thereto, without the presentation or surrender of this
       Certificate or the making of any notation hereon, provided,
       however, that in the event Certificates are issued in the form of
       Registered Certificates, distributions on this Certificate will
       be made by check mailed to the Person entitled thereto.  Except
       as otherwise provided in the Pass Through Trust Agreement and
       notwithstanding the above, the final distribution on this
       Certificate will be made after notice mailed by the Pass Through
       Trustee of the pendency of such distribution and only upon
       presentation and surrender of this Certificate at the office or
       agency of the Pass Through Trustee specified in such notice.

            THIS CERTIFICATE SHALL BE GOVERNED BY AND CONSTRUED IN
       ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK.

            Reference is hereby made to the further provisions of this
       Certificate set forth on the reverse hereof, which further
       provisions shall for all purposes have the same effect as if set
       forth at this place.

            Unless the certificate of authentication hereon has been
       executed by the Pass Through Trustee, by manual signature, this
       Certificate shall not be entitled to any benefit under the Pass
       Through Trust Agreement or be valid for any purpose.

            IN WITNESS WHEREOF, the Pass Through Trustee has caused this
       Certificate to be duly executed.


                                UNION TANK CAR COMPANY
                                _______ PASS THROUGH TRUST

                                By:   ___________________,
                                      as Pass Through Trustee


                                By: __________________________________________
                                    Title:


                                     A-3
 

   71


                            [Reverse of Certificate]



            The Certificates do not represent a direct obligation of, or
       an obligation guaranteed by (except to the extent provided in the
       Pass Through Trust Agreement), or an interest in Union, Procor,
       the Pass Through Trustee, in its individual or trust capacities,
       any Owner Participant, any Owner Trustee in its individual
       capacity, either Equipment Trust Trustee in its individual
       capacity, or any affiliate of any thereof.  The Certificates are
       limited in right of payment, all as more specifically set forth
       on the face hereof and in the Pass Through Trust Agreement.  All
       payments or distributions made to Certificateholders under the
       Pass Through Trust Agreement shall be made only from the Trust
       Property and only to the extent that the Pass Through Trustee
       shall have sufficient income or proceeds from the Trust Property
       to make such payments in accordance with the terms of the Pass
       Through Trust Agreement.  Each Certificateholder of this
       Certificate, by its acceptance hereof, agrees that it will look
       solely to the income and proceeds from the Trust Property to the
       extent available for distribution to such Certificateholder as
       provided in the Pass Through Trust Agreement.  This Certificate
       does not purport to summarize the Pass Through Trust Agreement
       and reference is made to the Pass Through Trust Agreement for
       information with respect to the interests, rights, benefits,
       obligations, proceeds, and duties evidenced hereby.  A copy of
       the Pass Through Trust Agreement may be examined during normal
       business hours at the principal office of the Pass Through
       Trustee, and at such other places, if any, designated by the Pass
       Through Trustee, by any Certificateholder upon request.

            The Pass Through Trust Agreement permits, with certain
       exceptions therein provided, the amendment thereof and the
       modification of the rights and obligations of the Companies and
       the rights of the Certificateholders under the Pass Through Trust
       Agreement at any time by the Companies and the Pass Through
       Trustee with the consent of the Certificateholders holding
       Certificates evidencing Fractional Undivided Interests
       aggregating not less than a majority in interest in the Pass
       Through Trust.  Any such consent by the Certificateholder of this
       Certificate shall be conclusive and binding on such
       Certificateholder and upon all future Certificateholders of this
       Certificate and of any Certificate issued upon the transfer
       hereof or in exchange hereof or in lieu hereof whether or not
       notation of such consent is made upon this Certificate.  The Pass
       Through Trust Agreement also permits the amendment thereof, in
       certain limited circumstances, without the consent of the
       Certificateholders of any of the Certificates.

            As provided in the Pass Through Trust Agreement and subject
       to certain limitations therein set forth, the transfer of this
       Certificate is registrable in the Register upon surrender of this
       Certificate for registration of transfer at the offices or
       agencies maintained by the Pass Through Trustee in its capacity
       as Registrar, or by any successor Registrar, in the Borough of
       Manhattan, the City of New York, duly endorsed or accompanied by
       a written instrument of transfer in form satisfactory to the Pass
       Through Trustee and the Registrar duly executed by the
       Certificateholder hereof or such Certificateholder's attorney
       duly authorized in writing, and thereupon one or more


                                     A-4

   72


       new Certificates of authorized denominations evidencing the same
       aggregate Fractional Undivided Interest in the Pass Through Trust
       will be issued to the designated transferee or transferees.

            The Certificates are issuable only as registered
       Certificates without coupons in minimum denominations of $1,000
       and $1,000 integral multiples thereof, except that one
       Certificate may be in a denomination of less than $1,000.  As
       provided in the Pass Through Trust Agreement and subject to
       certain limitations therein set forth, Certificates are
       exchangeable for new Certificates of authorized denominations
       evidencing the same aggregate Fractional Undivided Interest in
       the Pass Through Trust, as requested by the Certificateholder
       surrendering the same.

            No service charge will be made for any such registration of
       transfer or exchange, but the Pass Through Trustee shall require
       payment of a sum sufficient to cover any tax or governmental
       charge payable in connection therewith.

            The Pass Through Trustee, the Registrar, and any agent of
       the Pass Through Trustee or the Registrar may treat the person in
       whose name this Certificate is registered as the owner hereof for
       all purposes, and neither the Pass Through Trustee, the
       Registrar, nor any such agent shall be affected by any notice to
       the contrary.

            The obligations and responsibilities created by the Pass
       Through Trust Agreement and the Pass Through Trust created
       thereby shall terminate upon the distribution to
       Certificateholders of all amounts required to be distributed to
       them pursuant to the Pass Through Trust Agreement and the
       disposition of all property held as part of the Trust Property.


                                     A-5
 

   73


            [FORM OF THE PASS THROUGH TRUSTEE'S CERTIFICATE OF
       AUTHENTICATION]

       Dated:

                 This is one of the Certificates referred to in the
            within-mentioned Pass Through Trust Agreement.


                                _____________________,
                                as Pass Through Trustee



                                By:________________________________
                                    Authorized Officer



                                     A-6

   74


                                                                       EXHIBIT B

                                   GUARANTEE



            Union Tank Car Company, a Delaware corporation ("Union"),
       which term includes any successor or assign under the Pass
       Through Trust Agreement ______, dated _______, among Union,
       Procor Limited, a Canadian corporation ("Procor"), and
       ______________, as Pass Through Trustee, (the "Pass Through Trust
       Agreement"), has irrevocably and unconditionally guaranteed the
       due and punctual distribution to Certificateholders of principal
       and interest payable with respect to the Procor ETC (as defined
       in the Pass Through Trust Agreement), whether at stated maturity
       or otherwise, and certain other obligations of Procor as
       specified in Article XII of the Pass Through Trust Agreement and
       has agreed to pay any and all costs and expenses (including
       reasonable attorneys' fees) incurred by the Pass Through Trustee
       or any Certificateholder in enforcing any rights under this
       Guarantee.

            The obligations of Union to the Certificateholders and to
       the Pass Through Trustee pursuant to this Guarantee and the Pass
       Through Trust Agreement are expressly set forth in Article XII of
       the Pass Through Trust Agreement and reference is hereby made to
       such for the precise terms of this Guarantee.

            No stockholder, officer, director or incorporator, as such,
       past, present or future of Union shall have any liability under
       this Guarantee by reason of his or its status as such
       stockholder, officer, director or incorporator.

            This is a continuing Guarantee and shall remain in full
       force and effect and shall be binding upon Union and its
       successors and assigns until full and final payment of all of the
       guaranteed obligations under the Pass Through Trust Agreement and
       shall inure to the benefit of the successors and assigns of the
       Pass Through Trustee and the Certificateholders and, in the event
       of any transfer or assignment of rights by any Certificateholder
       or the Pass Through Trustee, the rights and privileges herein
       conferred upon that party shall automatically extend to and be
       vested in such transferee or assignee, all subject to the terms
       and conditions hereof.  This is a Guarantee of payment and not of
       collectibility.

            This Guarantee shall not be valid or obligatory for any
       purpose until the certificate of authentication on the
       Certificate upon which this Guarantee is noted shall have been
       executed by the Pass Through Trustee under the Pass Through Trust
       Agreement by the manual signature of one of its authorized
       officers.

            THE TERMS OF ARTICLE XII OF THE PASS THROUGH TRUST AGREEMENT
       ARE INCORPORATED HEREIN BY REFERENCE.


 

   75


                                                                       EXHIBIT C


                                  FORM OF DTC
                           LETTER OF REPRESENTATIONS



 

   76


                                   SCHEDULE I




                      Equipment Notes
                    to be Issued under                       Principal
                      the Indenture:                          Amount         Maturity
                    -------------------                      --------        --------
                                                                       
                   On the Issuance Date                              $

                     Subsequent to the
                       Issuance Date

                        Total Notes                                  $


      Equipment Trust Certificates to be issued under
                Equipment Trust Agreements

                   On the Issuance Date

                            UTC                                      $

                          Procor                                     $

                     Subsequent to the
                       Issuance Date

                            UTC                                      $

                        Total ETC's                                  $

                   Total Notes and ETC's                             $




 

   77



                                  SCHEDULE II




           ETCs
    to be Issued under                      Principal
Equipment Trust Agreements                   Amount                 Maturity
- --------------------------                  ---------               --------
                                                           
                                             $
          Total                              $











   78
Reconciliation and tie between Pass Through Trust Agreement _____ dated
________ and the Trust Indenture Act of 1939.  This reconciliation does not
constitute part of the Pass Through Trust Agreement.




Trust Indenture Act                                  Pass Through Trust
of 1939 Section                                      Agreement Section
- -------------------                                  ------------------
                                                     
                                                   
    310(a)(1) ...........................................  7.8
          (2) ...........................................  7.8
          (3) ...........................................  Inapplicable
          (4) ...........................................  6.2; 6.3 (a) & (b)
       (b)    ...........................................  7.8; 7.9; 7.10
       (c)    ...........................................  Inapplicable
    311(a)    ...........................................  7.18
       (b)    ...........................................  7.18
       (c)    ...........................................  Inapplicable
    312(a)    ...........................................  3.9; 8.1; 8.2
       (b)    ...........................................  13.4
       (c)    ...........................................  13.4
    313       ...........................................  8.3
    314(a)    ...........................................  8.4
       (b)    ...........................................  Inapplicable
       (c)(1) ...........................................  1.2
          (2) ...........................................  1.2
          (3) ...........................................  Inapplicable
       (d)(1) ...........................................  Inapplicable
          (2) ...........................................  Inapplicable
          (3) ...........................................  Inapplicable
       (e)    ...........................................  1.2
    315(a)    ...........................................  7.1(a)
       (b)    ...........................................  7.2
       (c)    ...........................................  7.1(b)
       (d)    ...........................................  7.1(c)
       (e)    ...........................................  6.6
    316(a)(last sentence) ...............................  1.4(c)
          (1)(A) ........................................  6.4
             (B) ........................................  6.5
          (2) ...........................................  Inapplicable
       (b)    ...........................................  6.7
       (c)    ...........................................  1.4(d)
    317(a)(1) ...........................................  6.3(a)
          (2) ...........................................  6.3(b)
       (b)    ...........................................  7.13
    318(a)    ...........................................  13.7