Only one railroad, the Union Pacific, put the 4-12-2 wheel arrangement into service and because of this the design was named after it.

Curiously, despite its troubles with the 4-10-2 Southern Pacific/Overland it had purchased from the American Locomotive Company (Alco) a year earlier which utilized three cylinders UP still chose to purchase a design that featured the same setup.

The railroad was after a more powerful locomotive, which could be used in main line freight service at higher speeds and apparently was willing to deal with the maintenance headache of three-cylinder steam.

During the late 1920s UP went on to roster nearly 100 examples of this interesting wheel arrangement and overall the railroad turned out to be relatively pleased with its operational service.

The 4-12-2s remained in use for nearly 30 years until they were finally retired in the 1950s. Today, one example is preserved.