EXHIBIT 31A

CERTIFICATION OF PRINCIPAL EXECUTIVE OFFICER

I, Ronald H. Zech, certify that:

     1. I have reviewed this Annual Report on Form 10-K of GATX Corporation;

     2. Based on my knowledge, this annual report does not contain any untrue
statement of a material fact or omit to state a material fact necessary to make
the statements made, in light of the circumstances under which such statements
were made, not misleading with respect to the period covered by this annual
report;

     3. Based on my knowledge, the financial statements, and other financial
information included in this annual report, fairly present in all material
respects the financial condition, results of operations and cash flows of the
registrant as of, and for, the periods presented in this annual report;

     4. The registrant's other certifying officer and I are responsible for
establishing and maintaining disclosure controls and procedures (as defined in
Exchange Act Rules 13(a)-15(e) and 15(d)-15(e)) for the registrant and have:

          (a) Designed such disclosure controls and procedures or caused such
     disclosure controls to be designed under our supervision to ensure that
     material information relating to the registrant, including its consolidated
     subsidiaries, is made known to us by others within these entities,
     particularly during the period in which this annual report is being
     prepared;

          (b) Evaluated the effectiveness of the registrant's disclosure
     controls and procedures and presented in this report our conclusions about
     the effectiveness of the disclosure controls and procedures, as of the end
     of the period covered by this report based on such evaluation; and

          (c) Disclosed in this report any change in the registrant's internal
     control over financial reporting that occurred during the registrant's most
     recent fiscal quarter (the registrant's fourth quarter in the case of an
     annual report) that has materially affected, or reasonably likely to
     materially affect, the registrant's internal control over financial
     reporting; and

     5. The registrant's other certifying officer and I have disclosed, based on
our most recent evaluation, to the registrant's auditors and the audit committee
of the registrant's board of directors (or persons performing the equivalent
function):

          (a) All significant deficiencies and material weaknesses in the design
     or operation of internal controls over financial reporting which are
     reasonably likely to adversely affect the registrant's ability to record,
     process, summarize and report financial information; and

          (b) Any fraud, whether or not material, that involves management or
     other employees who have a significant role in the registrant's internal
     control over financial reporting.

                                                  /s/ RONALD H. ZECH
                                          --------------------------------------
                                                      Ronald H. Zech
                                              Chairman, President and Chief
                                                    Executive Officer

March 15, 2004

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