4-12-2
Under the Whyte notation for the classification of steam locomotives, 4-12-2 represents the wheel arrangement of four leading wheels, twelve coupled driving wheels, and two trailing wheels. This arrangement was named the Union Pacific type, after the only railroad to use it, the Union Pacific Railroad.
Other equivalent classifications are:
AAR wheel arrangement: 2-F-1
UIC classification: 2′F1′
French classification: 261
Turkish classification: 69
Swiss classification: 6/9
Only one type of locomotive with a 4-12-2 wheel arrangement was built: the Union Pacific Railroad's 9000-series 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 inside third cylinder driving the cranked second driving axle between the frames. There was no inside 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 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 Walschaerts" valve gear which utilized a double eccentric crank and second link on the right side, 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 Gresley 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 design the third and fourth driving axles were planned to be "blind" in order to improve curve handling, but ALCO's lateral motion devices on the first and sixth axles made this unnecessary. They had the longest rigid wheelbase in North America, and the longest in the world until the Soviet Union built their 4-14-4 locomotive in 1934. The trailing truck carried the same axle load as the drivers, which was unusual.
There has been 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. Union Pacific drawings show no such crank on the first axle, and the Railway Age article says "The drivers permit the use of a straight axle on the front drivers..." The spacing between the first and second axles was increased by to provide clearance. Based on the published dimensions, this means at its closest the centerline of the inside rod was from the centerline of the first axle.
| Year | Quantity | Class | Alco order number | Alco serial numbers | Union Pacific Number | Notes |
| 1926 | 1 | UP-1 | B-1684 | 66544 | Union Pacific 9000 | Preserved |
| 1926 | 14 | UP-2 | B-1684 | 67024–67037 | Union Pacific 9001–9014 | 9004 to OWR&N 9708, then back to UP 9004 |
| 1928 | 15 | UP-3 | B-1706 | 67581–67595 | Union Pacific 9015–9029 | |
| 1928 | 8 | UP-3 | B-1708 | 67596–67603 | Oregon-Washington Railroad and Navigation Company 9700–9707 | to Union Pacific 9055–9062 |
| 1929 | 25 | UP-4 | S-1646 | 67944–67986 | Union Pacific 9030–9054 | |
| 1930 | 15 | UP-5 | S-1701 | 68490–68504 | Union Pacific 9063–9077 | to Oregon Short Line 9500–9514 |
| 1930 | 10 | UP-5 | S-1701 | 68505–68514 | Union Pacific 9078–9087 | |
| Total | 88 |
Only one example has survived into preservation. Union Pacific 9000 is preserved at the Railway and Locomotive Historical Society's museum at the Los Angeles County Fairplex in Pomona, California. It received a new boiler paint job in 2006-2007.