EXHIBIT 31B

CERTIFICATION OF PRINCIPAL FINANCIAL OFFICER

I, Robert C. Lyons, 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 13a-15(e) and 15d-15(e)) and internal
      control over financial reporting (as defined in Exchange Act Rules
      13a-15(f) and 15d-15(f)) 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
            quarterly report is being prepared;

            b) Designed such internal control over financial reporting, or
            caused such internal control over financial reporting to be designed
            under our supervision, to provide reasonable assurance regarding the
            reliability of financial reporting and the preparation of financial
            statements for external purposes in accordance with generally
            accepted accounting principles;

            c) 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

            d) 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 quarterly 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 of internal control over financial
      reporting, 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 control 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.

May 6, 2005

                                       /s/ Robert C. Lyons
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                                         Robert C. Lyons
                            Vice President and Chief Financial Officer

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