Exhibit 5.1



                    [MAYER, BROWN, ROWE & MAW LLP LETTERHEAD]



                                January 16, 2004





GATX Corporation
500 West Monroe Street
Chicago, Illinois  60661

GATX Financial Corporation
500 West Monroe Street
Chicago, Illinois  60661

Ladies and Gentlemen:

         We have acted as your counsel in connection with the registration of
$125,000,000 aggregate principal amount of 5% Senior Convertible Notes due 2023
(the "Notes") issued by GATX Corporation, a New York corporation (the
"Company"), and the shares of common stock, $.625 par value per share, of the
Company into which the Notes may be converted (the "Shares"). The Notes are
fully and unconditionally guaranteed (the "Guarantee") by GATX Financial
Corporation, a Delaware corporation (the "Guarantor"). The Notes were issued
pursuant to an indenture, dated as of August 15, 2003, by and among the Company,
the Guarantor and JPMorgan Chase Bank, as trustee. The Notes and the Shares may
be sold from time to time by the holders thereof.

         In rendering the opinions expressed herein, we have examined and relied
upon such documents, corporate records, certificates of public officials and
certificates as to factual matters executed by officers of the Company and the
Guarantor as we have deemed necessary or appropriate. We have assumed the
authenticity, accuracy and completeness of all documents, records, and
certificates submitted to us as originals, the conformity to the originals of
all documents, records and certificates submitted to us as copies and the
authenticity, accuracy and completeness of the originals of all documents,
records and certificates submitted to us as copies. We have also assumed the
legal capacity and genuineness of the signatures of persons signing all
documents in connection with which the opinions expressed herein are rendered.





MAYER, BROWN, ROWE & MAW LLP


GATX Corporation
GATX Financial Corporation
January 16, 2004
Page 2


         Based upon and subject to the foregoing and to the limitations set
forth herein, we are of the opinion that:

                  (i) The Notes have been legally issued and are binding
         obligations of the Company enforceable in accordance with their terms
         except as such enforceability may be limited by bankruptcy, insolvency,
         fraudulent conveyance, reorganization or similar laws affecting
         creditors' rights generally and subject to general principles of
         equity.

                  (ii) The Shares, when issued upon conversion of the Notes,
         will be legally issued, fully paid and non-assessable.

                  (iii) The Guarantee has been legally issued and is the binding
         obligation of the Guarantor, enforceable against the Guarantor in
         accordance with its terms, except as such enforceability may be limited
         by bankruptcy, insolvency, fraudulent conveyance, reorganization or
         similar laws affecting creditors' rights generally and subject to
         general principles of equity.

         We hereby consent to the filing of this opinion letter as an exhibit to
the registration statement covering resales of the Notes and the Shares and to
the reference to this firm under the caption "Legal Matters" contained therein.

                                                Very truly yours,

                                                /s/ MAYER, BROWN, ROWE & MAW LLP

                                                MAYER, BROWN, ROWE & MAW LLP