EXHIBIT 31B

CERTIFICATION OF PRINCIPAL FINANCIAL OFFICER

I, Brian A. Kenney, certify that:

1.    I have reviewed this Quarterly Report on Form 10-Q of GATX Corporation;

2.    Based on my knowledge, this quarterly 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 quarterly report;

3.    Based on my knowledge, the financial statements, and other financial
      information included in this quarterly 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
      quarterly 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 we 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
            quarterly 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
            fiscal quarter in the case of an annual report) that has materially
            affected, or is 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.

November 8, 2004

                                                /s/ Brian A. Kenney
                                   ---------------------------------------------
                                                  Brian A. Kenney
                                       President and Chief Financial Officer