The Union Pacific 9000 Class or "Union Pacific" type locomotive has a 4-12-2 wheel arrangement. It is the only steam locomotive to be built with that wheel arrangement. They featured three cylinders and used the conjugated valve gear invented in the United Kingdom by Sir Nigel Gresley.
The locomotives were used exclusively by the Union Pacific Railroad and were the largest steam locomotive class built on a rigid frame. It had a wheelbase of over 30 feet. The American Locomotive Company (ALCO) built a total of 88 locomotives in the class.
The locomotives had a top speed of 50 MPH and developed 96,646 lb of tractive effort. "Union Pacific" type locomotives were used primarily to increase the speed of freight trains across flat country. Despite some problems and very high maintenance requirements, the locomotives were very successful.
The prototype locomotive (#9000) survives today and is on display at the RailGiants Train Museum at the Los Angeles County Fairgrounds in Pomona, California.