Union Pacific 9000 Class

Ranked #150 on the list Best Locomotive class of All Time

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Based on 4 votes

About Union Pacific 9000 Class

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The Union Pacific 9000 Class was a class of 88 4-12-2 steam locomotives built by Alco between 1926 and 1930. Union Pacific's crossing of the continental divide had steep grades, and train tonnages were constantly increasing. During the 1920s, trains were being handled by 2-8-8-0 Mallet compounds and 2-10-2s. Speeds above 25 mph were not then possible with the Mallet arrangement, whilst 2-10-2s had limited adhesion. To enable a 12-coupled locomotive to negotiate curves, the 9000s were provided with Alco's new lateral motion device. These locomotives were used to increase the speed of freight trains in flat country, and were fairly successful, but were maintenance nightmares, largely because of their use of an inaccessible third cylinder driving a cranked second driving axle between the frames. There was no inaccessible valve gear to worry about, however. ALCO had obtained permission to use the conjugated valve gear invented by Sir Nigel Gresley. This system used two hinged levers connected to the outer cylinder's valves to operate the inner cylinder's valve. The 9000 class locomotives were the largest ever to use Gresley gear. Between 1934 and 1940 eight of the first fifteen locos

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