Material Agreement, Financial Obligation (Form 8-K)

Item 1.01 Entry Into A Material Definitive Agreement

GATX Corporation ("GATX") entered into an Underwriting Agreement (the "Underwriting Agreement"), with BofA Securities, Inc., Citigroup Global Markets Inc. and Morgan Stanley & Co. LLC, as representatives of the several underwriters listed therein (collectively, the "Underwriters"), dated October 22, 2025, pursuant to which GATX agreed to sell and the Underwriters agreed to purchase, subject to and upon terms and conditions set forth therein, $200,000,000 aggregate principal amount of 5.500% Senior Notes due 2035 (the "2035 Notes") and $200,000,000 aggregate principal amount of 6.050% Senior Notes due 2054 (the "2054 Notes" and, together with the 2035 Notes, the "Notes"), as described in the prospectus supplement, dated October 22, 2025 (the "Prospectus Supplement"), filed pursuant to GATX's shelf registration statement on Form S-3,Registration No. 333-286436. The2035 Notes will form a single series with and have the same terms as the $500,000,000 aggregate principal amount of 2035 Notes issued by GATX on February 6, 2025. Upon completion of this offering, an aggregate $700,000,000 of GATX's 2035 Notes will be outstanding. The 2054 Notes will form a single series with and have the same terms as the $400,000,000 aggregate principal amount of 2054 Notes issued by GATX on June 5, 2024 and the $300,000,000 aggregate principal amount of 2054 Notes issued by GATX on February 6, 2025. Upon completion of this offering, an aggregate $900,000,000 of GATX's 2054 Notes will be outstanding.

The Notes were issued under the Indenture, dated as of February 6, 2008, between GATX and U.S. Bank Trust Company, National Association, as trustee, as successor in interest to U.S. Bank National Association, and the officers' certificates providing for the issuance of the Notes. The Underwriters delivered the Notes against payment on October 24, 2025.

The above description of the Underwriting Agreement is a summary and is not complete. The above summary is qualified by reference to the terms of the full Underwriting Agreement. Copies of the Underwriting Agreement and other documents relating to this transaction are attached as exhibits to this Current Report on Form 8-Kand are incorporated herein by reference.

Item 2.03. Creation of a Direct Financial Obligation or an Obligation under an Off-BalanceSheet Arrangement of a Registrant

See Item 1.01.