Union Pacific 4-12-2 No. 9085 Alco builder's photo.

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(American Locomotive Company, W. Lenheim Collection)


4-12-2 UP 9000 CLASS

The Union Pacific Railroad 9000 Class was a class of 88 steam locomotives, built by ALCO for the Union Pacific between 1926 and 1930.

The 4-12-2 wheel arrangement. Front of locomotive on left.

(Gwernol, Public domain, via Wikimedia Commons)


Wheel arrangement

The Union Pacific 9000 class was the only class of steam locomotives with a 4-12-2 wheel arrangement ever to be built. Under the Whyte notation for the classification of steam locomotives, the locomotives had four leading wheels, twelve coupled driving wheels, and two trailing wheels. As the Union Pacific was the only operator of this wheel arrangement, it was often nicknamed the Union Pacific type.

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
Russian classification: 2-6-1


The prototype, UP 9000, as preserved at the Los Angeles County Fairplex in Pomona, California. (Morven, Creative Commons Attribution-Share Alike 3.0 Unported, via Wikimedia Commons) 

Front view of the same locomotive. The third cylinder and the mechanism that controls it can be seen below the smokebox. (Morven, Creative Commons Attribution-Share Alike 3.0 Unported, via Wikimedia Commons) 



These locomotives were used to increase the speed of freight trains in flat country, and were fairly successful, but had very high maintenance requirements, 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, however. ALCO had obtained permission to use the conjugated valve gear invented in the UK by Sir Nigel Gresley, who was the chief mechanical engineer of the London and North Eastern Railway. 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 gears.

Between 1934 and 1940, eight of the first fifteen locomotives 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 three-cylinder experimental compound 4-10-2 No. 60000), which operated the valve for the inside cylinder. Union Pacific referred to this system as the "third link". The 4-12-2s 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 AA20 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. Based on the published dimensions, 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.)


View of the UP's locomotive number 9000 in Popular Mechanics, August 1927.

(Oaktree, Popular Mechanics, Public domain. via Wikimedia Commons)


Union Pacific UP classes

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

Preserved Example

Only one example has survived into preservation. Union Pacific 9000, 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 new boiler paint in 2006-2007.


Photo of Union Pacific 9043, a 4-12-2 locomotive, pulling freight near Omaha. This was an UP class 4 locomotive, built by Alco in 1929.

(Bob Fremming-Dallas, Wisconsin, Public domain, via Wikimedia Commons)


Type and origin

Power type: Steam
Builder: American Locomotive Company
Build date: 1926–1930
Total produced: 88
​• Whyte 4-12-2
• UIC 2′F1′ h3
Gauge: 4 ft 8+1⁄2 in (1,435 mm)
Driver diameter: 67 in (1,702 mm)
Wheelbase Locomotive & 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)
Locomotive 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 capacity: 18,000 US gallons (68,000 L; 15,000 imp gal)
• Firegrate area 108.25 sq ft (10.057 m2)
Boiler: 92 in (2,337 mm)
Boiler pressure: 220 lbf/in2 (1.52 MPa)
Heating surface:
• Firebox 591 sq ft (54.9 m2)
​• Heating area 2,560 sq ft (238 m2)
Cylinders: Three
Cylinder size - Outside (2): 27 in × 32 in (686 mm × 813 mm);
Cylinder size - Inside (1): 27 in × 31 in (686 mm × 787 mm)
Valve gear: Gresley Conjugated
Performance figures
Maximum speed: 50 mph (80 km/h)
Tractive effort: 96,646 lbf (429.9 kN)
Factor of adhesion: 3.66
Operator: 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