GATX Corporation (NYSE: GATX) is an equipment finance company based in Chicago, Illinois. Founded in 1898, GATX's primary activities consist of railcar operating leasing in North America and Europe. In addition, GATX leases locomotives in North America, and also has significant investments in industrial equipment and marine assets, including ownership of the American Steamship Company, which operates on the Great Lakes. The CEO/Chairman is Brian A. Kenney.

GATX Corporation is divided into four business segments: Rail North America, Rail International, American Steamship Company (ASC), and Portfolio Management. Portfolio Management consists largely of the corporation's non-rail and non-Great Lakes assets.

GATX is one of several major North American rail operating lessors, and measured by fleet size, ranks as number two in this market behind GE Rail Services. Other major North American rail operating lessors include CIT, First Union, Union Tank Car Company, Trinity Industries Leasing, ARL, and Helm Financial.

GATX derives its name from its primary reporting mark for its North American railcars, "GATX". The mark itself was derived from GATX's prior corporate name, "General American Transportation". History includes GATX working with famous designer Russel Wright to develop "Meladur", a famous melamine dinnerware from 1943-1945, using Melmac by American Cyanamid which was to be used on passenger cars and in hospitals marked with the name "General American". Since all non-railroad owners of railcars must append an "X" to the end of their mark, GAT became GATX. GATX mainly applies the GATX mark to tank cars, although the mark has been used in other examples such as with hoppers; GATX's primary freightcar marks are GACX (for general-service freight cars), GGPX (for coal cars), GIMX (for intermodal cars), GPLX (for plastic pellet cars), GMTX and LLPX (for locomotives), and GPFX (for pressure-differential cars). GATX also owns a number of other marks, including GABX, GAEX, GFSX, GOHX, GSCX, IPSX, and TRIX. Many GATX cars carry a large "GATX" logo in the upper right-hand corner of the car regardless of the reporting mark they carry; this logo is applied for marketing reasons and does not have any operational significance.

The General American Transportation Corporation became GATX Rail Corporation, a unit of the GATX Corporation, on January 1, 2000.

GATX engages in both full-service and net leasing of railcars. In a full-service lease, a GATX-owned mark is applied to the car, and GATX maintains the railcar and pays for any required property insurance and property taxes. In a net lease, the lessee applies its mark to the car, and the lessee pays for any required property insurance and property taxes. Often, on a net-leased car, there is no evidence of GATX ownership, although some net lease cars carry a GATX logo.

The most common type of car in the GATX North American fleet is the tank car; other major car types include covered hoppers, open-top hoppers, and gondolas. GATX invests in nearly every type of railcar operated in North America. In Europe, tank cars also make up GATX's largest fleet, but unlike in North America, GATX's European fleet includes substantial quantities of intermodal cars which are owned in a GATX joint venture called AAE Cargo. In contrast, GATX's North American intermodal car fleet is relatively small. This is true of most North American operating lessors; historically the bulk of the industry's intermodal investment has been made by TTX Corporation, which is jointly owned by North America's Class I railroads.