EXHIBIT 10.1

                                AMENDMENT NO. 4
                                ---------------



          THIS AMENDMENT NO. 4 (this "Agreement") is entered into as of December
22, 1997 by and among ABC Rail Products Corporation ("Rail"), ABC Deco Inc.
("Deco"), American System Technologies, Inc. ("AST;" AST, Rail and Deco being
the "Borrowers"), the financial institutions named on the signature pages hereto
(collectively, the "Lenders") and American National Bank and Trust Company of
Chicago, as agent for the Lenders (the "Agent").

                                    RECITALS
                                    --------

          A.  The Agent, the Lenders and the Borrowers have entered into a 
Second Amended and Restated Loan and Security Agreement dated as of January 31,
1997 (as heretofore amended, supplemented or otherwise modified, the "Loan
Agreement").

          B.  The Borrowers desire that Rail issue subordinated indebtedness in
an aggregate principal amount of up to $25,000,000 (the "Subordinated Debt").

          C.  In order to permit the issuance of the Subordinated Debt, the 
Borrowers have requested that the Agent and the Lenders agree to amend the Loan
Agreement pursuant to the terms and subject to the conditions hereof.

          D.  The Agent and the Lenders are willing to enter into this 
Agreement, but only on the terms and subject to the conditions set forth below.

          NOW, THEREFORE, in consideration of the premises set forth above, the
terms and conditions contained herein, and other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged, the
parties hereto agree as follows:

          1.   Definitions.  Terms defined in the Loan Agreement which are used
herein shall have the same meaning as are set forth in the Loan Agreement for
such terms unless otherwise defined herein.

          2.   Amendments.  Subject to Section 3 below:

               (a)  The following definition set forth in Subsection 1.1 is
     hereby amended and restated as follows:

                    "Final Termination Date" shall mean the earlier of (a) March
          31, 2005, and (b) the date that is ninety-one (91) days prior to the
          earliest stated maturity of any Subordinated Notes.

 
               (b)  Subsection 1.1 is hereby further amended by adding thereto
     (after the definition of Applicable LIBOR Margin) the following:

                    "'Approved Officer's Certificates' shall mean, collectively,
          the certificates attached to this Agreement as Exhibits J-1 and J-2."

               (c)  Subsection 8.2(viii) is hereby amended by deleting reference
     therein to "$50,000,000" and inserting in lieu thereof: "$75,000,000."

               (d)  The last sentence of Subsection 8.17 is hereby amended and
     restated as follows: "Rail shall not, without the prior written consent of
     the Required Lenders: (X) consent or otherwise agree to any amendment,
     supplement or other modification to any Subordinated Debt Document or (Y)
     execute or deliver any "Officer's Certificate" (as defined in the
     Subordinated Debt Indenture) pursuant to Section 301 of the Subordinated
     Debt Indenture or otherwise establish the terms of any "Securities" (as
     defined in the Subordinated Debt Indenture) under the Subordinated Debt
     Indenture other than the Approved Officer's Certificates.

               (e)  The Loan Agreement is further amended by adding immediately
     after Exhibit J thereto an Exhibit J-1 and an Exhibit J-2 in the form of
     Exhibit J-1 and Exhibit J-2 hereto, respectively.

          3.   Conditions.  The terms of Section 2 above shall become effective
only when each of the following conditions have been completely satisfied as
determined by the Agent in its sole discretion (the date of such satisfaction
being hereinafter referred to as the "Effective Date"):

               (a)  Documents.  The Agent shall have received each of the
     following agreements, instruments and other documents, in each case in form
     and substance acceptable to the Agent:

                    (i) eight (8) copies of this Agreement duly executed and
          delivered by each of the Borrowers and the Required Lenders;

                    (ii) eight (8) copies of a Reaffirmation Agreement duly
          executed and delivered by each of the Borrowers and certain of their
          affiliates in the form of Exhibit A attached hereto;

                    (iii) eight (8) copies of a certificate duly executed and
          delivered by the chief financial officer of each Borrower as to (1)
          all representations and warranties of each Borrower in the Loan
          Agreement (both immediately before and after giving effect to this
          Agreement and the issuance of the Subordinated Debt (collectively, the
          "Contemplated Transactions") being true and complete, (2) the absence
          of any Default or Event of Default (both immediately before and after
          giving effect to the

                                      -2-

 
          Contemplated Transactions), and (3) with respect to Rail only, the
          Subordinated Debt Documents being in full force and effect and being
          in the form attached to such certificate;

                    (iv) eight (8) copies of a certificate duly executed by the
          corporate secretary of each Borrower as to: (1) the resolutions
          adopted by its Board of Directors (and, if necessary, its
          shareholders) authorizing the execution, delivery and performance of
          this Agreement and the other agreements and instruments contemplated
          hereby and thereby, (2) the incumbency, names and signatures of the
          officers of such Borrower who are duly authorized to execute and
          deliver the foregoing items, and (3) the Charter and By-Laws of each
          such Borrower;

                    (v) certificates of good standing as to each Borrower issued
          by the appropriate state office(s) in the jurisdiction of such
          Borrower's incorporation, dated as of a date within five (5) days
          prior to the Effective Date;

                    (vi) eight (8) copies of an opinion of Jones, Day, Reavis &
          Pogue, special counsel to the Borrower ("Jones Day"), in the form of
          Exhibit B attached hereto;

                    (vii) eight (8) copies of a reliance letter of Jones Day
          which provides, in effect, that the Agent and the Lenders (including
          the Issuing Bank) may fully rely on the opinion letter or letters (as
          the case may be) rendered by Jones Day in connection with the issuance
          of the Subordinated Debt; and

                    (viii) such other documents, certificates, agreements,
          opinions and items as the Agent may request in connection with the
          Contemplated Transactions.

               (b)  Subordinated Debt.  All indentures, agreements, instruments
     and documents relating to the Subordinated Debt (the "Subordinated Debt
     Documents") shall, in all respects be satisfactory in form and substance to
     the Agent and the Lenders, as determined in their sole and absolute
     discretion; such Subordinated Debt Documents shall be in full force and
     effect, and no default (or event which with the passage of time, giving of
     notice or both would constitute a default) shall exist thereunder; and Rail
     shall have concurrently received not less than $24,125,000 net cash
     proceeds from the issuance of the Subordinated Debt.

               (c)  Representations and Warranties; No Default.  As of the
     Effective Date, the representations and warranties contained herein and in
     the Loan Agreement (both immediately before and after giving effect to the
     Contemplated Transactions) shall be true and complete, and no Default or
     Event of Default shall exist.

               (d)  Proceedings.  All corporate and other proceedings taken or 
     to be taken in connection with the transactions contemplated hereby and all
     documents incident thereto

                                      -3-

 
     shall be in form and substance satisfactory to the Agent and the Required
     Lenders, as determined in their sole and absolute discretion.

          4.   Representations, Warranties and Agreements of the Borrowers.

               (a)  Each of the Borrowers represents and warrants that: (1) the
     execution and delivery by such Borrower of this Agreement and the
     agreements and instruments contemplated hereby and the performance of each
     Borrower's obligations hereunder and thereunder: (i) are within the
     corporate powers of each Borrower; (ii) are duly authorized by the Board of
     Directors of each Borrower, and, if necessary, the stockholders of each
     Borrower; (iii) are not in contravention of the terms of the Charter or By-
     Laws of either Borrower, or of any contract, instrument, indenture or other
     agreement or undertaking to which either Borrower is a party or by which
     either Borrower or any of its property is bound or any judgment, decree or
     order applicable to either Borrower; (iv) do not require any governmental
     consent, registration or approval or any filing with or notice to any
     governmental entity or agency; (v) do not contravene any governmental
     restriction binding upon either Borrower; and (vi) will not result in the
     imposition of any lien, charge, security interest or encumbrance upon any
     property of either Borrower under any indenture, mortgage, deed of trust,
     loan or credit agreement or other agreement or instrument to which either
     Borrower is a party or by which it or any of its property may be bound or
     affected; (2) this Agreement has been duly executed and delivered by each
     Borrower and constitutes the legal, valid and binding obligation of each
     Borrower, enforceable against each Borrower in accordance with its terms,
     except as limited by applicable bankruptcy, reorganization, insolvency or
     similar laws affecting the enforcement of creditors' rights generally and
     except as limited by general principles of equity; (3) the Loan Agreement,
     after giving effect hereto, constitutes  the legal, valid and binding
     obligation of the Borrowers enforceable against the Borrowers in accordance
     with its terms, except as enforcement may be limited by bankruptcy,
     insolvency, reorganization or other similar laws affecting the enforcement
     of creditors' rights generally or by general equitable principles; and (4)
     as of the date hereof, and as of the Effective Date (both immediately
     before and after giving effect to the Contemplated Transactions), there
     exists no Default or Event of Default.

               (b)  Each of the Borrowers hereby reaffirms all covenants,
     representations and warranties made in the Loan Agreement and all other
     Financing Agreements.  Each of the Borrowers hereby agrees that all
     covenants, representations and warranties made in the Loan Agreement and
     all other Financing Agreements shall be deemed to have been remade as of
     the date hereof and the Effective Date.

          5.   Reference to the Effect on the Loan Agreement.

                                      -4-

 
               (a)  On and after the Effective Date, (i) each reference in the
     Loan Agreement to "this Agreement," "hereunder," "hereof," "herein," or
     words of like import shall mean and be a reference to the Loan Agreement as
     amended hereby, and (ii) each reference to the Loan Agreement in all other
     Financing Agreements shall mean and be a reference to the Loan Agreement,
     as amended hereby.

               (b)  Except as specifically amended above, the Loan Agreement, 
     and all other Financing Agreements and other documents, instruments and
     agreements executed and/or delivered in connection therewith, shall remain
     in full force and effect, and are hereby ratified and confirmed.

               (c)  The execution, delivery and effectiveness of this Agreement
     shall not operate as a waiver of any right, power or remedy of the Agent or
     the Lenders, nor constitute a waiver of, or consent to and departure from,
     any provision of the Loan Agreement, any other Financing Agreement, or any
     other documents, instruments and agreements executed and/or delivered in
     connection therewith.

          6.   Governing Law.  This Agreement shall be governed by and construed
in accordance with the internal laws (without giving effect to conflicts of law
principles) of the State of Illinois.

          7.   Headings.  Section headings in this Agreement are included herein
for convenience of reference only and shall not constitute a part of this
Agreement for any other purpose.

          8.   Counterparts.  This Agreement may be executed by one or more of
the parties to this Agreement on any number of separate counterparts and all of
said counterparts taken together shall be deemed to constitute one and the same
instrument.  Delivery by any party of an executed counterpart hereof by telecopy
of similar facsimile transmission shall constitute valid and effective delivery
hereof by such party.

          9.   Termination.  This Agreement shall cease to be of any effect if
the Effective Date has not occurred on or before December 31, 1997.

                                      -5-

 
          IN WITNESS WHEREOF, this Agreement has been duly executed as of the
day and year first above written.

                              ABC RAIL PRODUCTS CORPORATION
                         
                                  /s/ D. CHISHOLM MACDONALD
                              By:_________________________________________
                                  EXECUTIVE VICE-PRESIDENT--ADMINISTRATION 
                                  AND BUSINESS DEVELOPMENT
                              Title:______________________________________

                              ABC DECO INC.

                                  /s/ D. CHISHOLM MACDONALD
                              By:_________________________________________

                              Title:______________________________________

                              AMERICAN SYSTEMS TECHNOLOGIES, INC.

                                  /s/ D. CHISHOLM MACDONALD
                              By:_________________________________________

                              Title:______________________________________

                              AMERICAN NATIONAL BANK AND TRUST
                              COMPANY OF CHICAGO, individually and as
                              Agent

                                  /s/ DENNIS E. HARRISON
                              By:_________________________________________
                                     SENIOR VICE PRESIDENT
                              Title:______________________________________

                              BTM CAPITAL CORPORATION

                                  /s/ WILLIAM R. YORK, JR.
                              By:_________________________________________
                                     Senior Vice President
                              Title:______________________________________

 
                              LASALLE NATIONAL BANK

                                  /s/ TERRI MAURER
                              By:_________________________________________
                                     VICE PRESIDENT
                              Title:______________________________________

                              NATIONS BANK OF TEXAS, N.A.

                                  /s/ JAMES BERKEMEIER
                              By:_________________________________________
                                     VICE PRESIDENT
                              Title:______________________________________

                              MELLON BANK, N.A.

                                  /s/ JEFFREY G. SAPERSTEIN
                              By:_________________________________________
                                     ASSISTANT VICE PRESIDENT
                              Title:______________________________________

 
                                                                       EXHIBIT A
                                                                       ---------



                                 See Attached.

 
                            REAFFIRMATION AGREEMENT
                            -----------------------


          This Reaffirmation Agreement ("Reaffirmation") is made as of December
23, 1997 by each of the undersigned (collectively, the "Undersigned") in favor
of American National Bank and Trust Company of Chicago, as Agent.


                                    RECITALS
                                    --------


          A.   The Undersigned are parties to one or more Financing Agreements
(as defined in the Loan Agreement referenced below) in connection with that
certain Second Amended and Restated Loan and Security Agreement dated as of
January 31, 1997 (as amended, supplemented or otherwise modified, the "Loan
Agreement"; terms defined in the Loan Agreement have the same meaning herein
unless otherwise defined).

          B.   The Undersigned desire that the Agent and the Lenders enter into
that certain Amendment No. 4 dated as of December 22, 1997 among the Agent, the
Borrowers and the Lenders (the "Amendment").

          C.   The Agent and the Lenders are willing to enter into the Amendment
only if, among other things, the Undersigned execute and deliver this
Reaffirmation.

          NOW, THEREFORE, in consideration of the facts recited above, to induce
the Agent and the Lenders to enter into the Amendment and for other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the Undersigned each agree as follows:

          1.   The Undersigned each hereby ratify and reaffirm  all of their
respective obligations and liabilities arising under, or relating to, each
Financing Agreement to which it is a party, in each case, after giving effect to
the Amendment, and each of the Undersigned further agree that each Financing
Agreement to which it is a party shall remain in full force and effect in
accordance with its terms, after giving effect to the Amendment.

          2.   Without limiting the foregoing, the term "Loan Agreement" and
each similar reference to the Loan Agreement as used in each Financing Agreement
shall mean and include the Loan Agreement as amended by the Amendment.

          3.   The execution, delivery and effectiveness of the Amendment and
the agreements and instruments contemplated thereby shall not diminish, or
operate as a waiver of, any right, power or remedy of the Agent or any Lender
under any Financing Agreement except to the extent of the waivers expressly set
forth in the Amendment.

          4.   Notice of acceptance hereof is hereby waived by the Undersigned.

 
          IN WITNESS WHEREOF, this Reaffirmation has been duly executed and
delivered as of the date first above written.

                              ABC RAIL PRODUCTS CORPORATION

                                  /s/ D. CHISHOLM MACDONALD
                              By:_________________________________________

                              Title:______________________________________

                              ABC RAIL PRODUCTS CHINA INVESTMENT
                              CORPORATION

                                  /s/ D. CHISHOLM MACDONALD
                              By:_________________________________________

                              Title:______________________________________

                              ABC RAIL BRAKE SHOE HOLDINGS, INC.

                                  /s/ D. CHISHOLM MACDONALD
                              By:_________________________________________

                              Title:______________________________________

                              ABC DECO INC.

                                  /s/ D. CHISHOLM MACDONALD
                              By:_________________________________________

                              Title:______________________________________

                              AMERICAN SYSTEMS TECHNOLOGIES, INC.

                                  /s/ D. CHISHOLM MACDONALD
                              By:_________________________________________

                              Title:______________________________________

                              UNITED RAILWAY SIGNAL GROUP, INC.

                                  /s/ D. CHISHOLM MACDONALD
                              By:_________________________________________

                              Title:______________________________________