Other equivalent classifications are: These locomotives were used to increase the speed of freight trains in flat country, and were fairly successful, but had very high maintenance requirements, largely because of their use of an inside third cylinder driving the cranked second driving axle between the frames.

ALCO had obtained permission to use the conjugated valve gear invented in the UK by Sir Nigel Gresley, who was the chief mechanical engineer of the London and North Eastern Railway.

The pre-1928 engines not converted received the roller bearing levers in 1940, and no further conversions were made.

The trailing truck carried the same axle load as the drivers, which was unusual.

Union Pacific 9000, the prototype of the class, is preserved at the Railway and Locomotive Historical Society's museum at the Los Angeles County Fairplex in Pomona, California.