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Description
From 1926 to 1930, the Union Pacific Railroad received 88 of a new and extreme class of "Big Engine" from Alco. This class, known as the 9000 class after the class leader, was a 4-12-2 steam engine, the longest rigid-wheelbase steam engine in America.* The 4-12-2 was developed to pull heavy freight trains at high speeds over long, straight, flat stretches of track. To increase the power, Alco fitted the 4-12-2 locomotive with a third cylinder, located under the boiler between the other two cylinders.

The 4-12-2 also was fitted with a Gresley valve gear, and became the largest locomotive to use this device. A lateral motion device allowed the axles to move sideways a few inches, but the relatively stiff 4-12-2 was still not good for mountain lines. The maneuverability problems with the 4-12-2 prompted the UP to develop articulated locomotives, and it did, resulting in the Challenger and Big Boy. No. 9000 is preserved in Pomona, California, the only surviving member of the only 4-12-2 class in the world.

* There was one rigid-frame locomotive that was larger, the Soviet experimental 4-14-4 "Andrei Andreev," which was not successful. So the American 4-12-2 was the largest rigid-frame locomotive to have been in series production.

Description
From 1926 to 1930, the Union Pacific Railroad received 88 of a new and extreme class of "Big Engine" from Alco. This class, known as the 9000 class after the class leader, was a 4-12-2 steam engine, the longest rigid-wheelbase steam engine in America.* The 4-12-2 was developed to pull heavy freight trains at high speeds over long, straight, flat stretches of track. To increase the power, Alco fitted the 4-12-2 locomotive with a third cylinder, located under the boiler between the other two cylinders.

The 4-12-2 also was fitted with a Gresley valve gear, and became the largest locomotive to use this device. A lateral motion device allowed the axles to move sideways a few inches, but the relatively stiff 4-12-2 was still not good for mountain lines. The maneuverability problems with the 4-12-2 prompted the UP to develop articulated locomotives, and it did, resulting in the Challenger and Big Boy. No. 9000 is preserved in Pomona, California, the only surviving member of the only 4-12-2 class in the world.

* There was one rigid-frame locomotive that was larger, the Soviet experimental 4-14-4 "Andrei Andreev," which was not successful. So the American 4-12-2 was the largest rigid-frame locomotive to have been in series production.