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.

Union Pacific 9000-series
The prototype, UP 9000, as preserved at the Los Angeles County Fairplex in Pomona, California.
Type and origin Power type Steam Builder American Locomotive Company Build date 1926–1930 Total produced 88
Specifications Configuration: ​  • Whyte 4-12-2  • UIC 2′F1′ h3 Gauge 4 ft 8 1⁄2 in (1,435 mm) Driver dia. 67 in (1,702 mm) Wheelbase Loco & tender: 91.50 ft (27.89 m) Length 102 ft 7 in (31.27 m) Axle load 59,000 lb (26,762 kg; 27 t) Adhesive weight 354,000 lb (160,572 kg; 161 t) Loco weight 496,500 lb (225,209 kg; 225 t) Tender weight 310,599 lb (140,885 kg; 141 t) Total weight 807,099 lb (366,094 kg; 366 t) Fuel type Coal Fuel capacity 42,000 lb (19,051 kg; 19 t) Water cap 18,000 US gallons (68,000 l; 15,000 imp gal) Firebox:  • Firegrate area 108.25 sq ft (10.057 m2) Boiler pressure 220 lbf/in2 (1.52 MPa) Heating surface:  • Firebox 591 sq ft (54.9 m2) Superheater: ​  • Heating area 2,560 sq ft (238 m2) Cylinders Three Cylinder size Outside (2): 27 in × 32 in (686 mm × 813 mm); Inside (1): 27 in × 31 in (686 mm × 787 mm)
Performance figures Tractive effort 97,664 lbf (434.4 kN) Factor of adh. 3.63
Career Operators Union Pacific Railroad Class UP-1 through UP-5 Numbers 9000–9087 Retired 1953–1956 Preserved One preserved (No. 9000), remainder scrapped Disposition No. 9000 on static display at the RailGiants Train Museum

Other equivalent classifications are:
AAR wheel arrangement: 2-F-1
UIC classification: 2′F1′ (also known as German classification and Italian classification)
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.

In this front view of the same locomotive the third cylinder and the mechanism that controlled it can be seen below the smokebox.

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 (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 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" (flangeless) in order to improve curve handling, but 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 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 67 in (170 cm) drivers permit the use of a straight axle on the front drivers..." The spacing between the first and second axles was increased by 18 in (46 cm) to provide clearance. Based on the published dimensions, this means at its closest the centerline of the inside rod was 11.645 in (29.58 cm) from the centerline of the first axle. (UP drawings reproduced in Kratville and Bush's "Union Pacific Type" books show the inside rod 113 in (290 cm) long and the first and second driver axles 88 in (220 cm) apart. The inside cylinder axis was inclined 9.5 degrees and was 32 in (81 cm) above the plane of the driving axles at a point 181 in (460 cm) ahead of the second driving axle, so the cylinder axis missed the centerline of the second axle by 1-11/16 inches. The rod centerline is closest to the axle when the crank is 54.49 degrees below horizontal.)

Only one example has survived into preservation. Union Pacific 9000 which is the Prototype of the class, 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. Passengers can go inside the cab where they can blow 9000’s U.P. star brass 5 chime steam whistle (now air compressed.)