Only one type of 4-12-2 was built, the Union Pacific Railroad's 9000 class locomotives, 88 of which were built by ALCO between 1926 and 1930. 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 so-called conjugated valve gear invented by Nigel Gresley. This system used two hinged rods connected to the outer cylinders to operate the inner cylinder. The 9000 class locomotives were the largest ever to use Gresley gear.
Between 1934 and 1940 eight of the first fifteen locos had their Gresley gear removed and were converted to a "double Walschearts" valve gear which utilized a double eccentric (return) crank and second link on the right side (similar to the gear Baldwin used on its 3-cylinder experimental compound 4-10-2 #60000), which operated the valve for the inside cylinder. Union Pacific referred to this system as the "third link." The 4-12-2's constructed from 1928 utilized roller bearings in the Gresely lever bearings, thus none of these engines were converted. The pre-1928 engines not converted received the roller bearing levers in 1940, and no further conversions were made.
During the design phase "blind" (flangeless) third and fourth axle were provided in order to improve curve handling, but the use of ALCO's lateral motion devices on the first and sixth axles (which allowed the axles to slide up to two inches to the side) made this unnecessary. They were the longest rigid frame locomotives in North America, and until 1934, the longest in the world. The trailing truck carried the same axle load as the drivers, which was unusual.
There has been some debate as to whether the first driving axle of the 4-12-2 was cranked to provide clearance for the main rod connected to the second axle. It has been shown that there was no such crank on the first axle. Instead, the spacing between the first and second axles was increased by 18 inches to provide the necessary clearance.
One example survives, at the Railway and Locomotive Historical Society's museum at the Los Angeles County Fairplex in Pomona, California.