Under the Whyte notation for the classification of steam locomotives, 2-10-2 represents the wheel arrangement of two leading wheels on one axle, usually in a leading truck, ten powered and coupled driving wheels on five axles, and two trailing wheels on one axle, usually in a trailing truck. In the United States and elsewhere the 2-10-2 is known as the Santa Fe type, after the Atchison, Topeka and Santa Fe Railway that first used the type in 1903.
Other equivalent classifications are:
- UIC classification: 1E1 (also known as German classification and Italian classification)
- French classification: 151 (also known as Spanish classification)
- Turkish classification: 57
- Swiss classification: 5/7
- Russian classification: 1-5-1
The 2-10-2 wheel arrangement evolved in the Unites States of America from the 2-10-0 Decapod of the Atchison, Topeka and Santa Fe Railway (ATSF). Their existing 2-10-0 locomotives, used as pushers up Raton Pass, encountered problems reversing back down the grade for their next assignments since they were unable to track around curves at speed in reverse and had to run very slowly to avoid derailing. Consequently, the ATSF added a trailing truck to the locomotives which allowed them to operate successfully in both directions. These first 2-10-2 locomotives became the forerunners to the entire 2-10-2 family.
The trailing truck allows a larger, deeper firebox than that of a 2-10-0. Like all ten-coupled designs, the long rigid wheelbase of the driving wheels presented a problem on curves, requiring blind drivers, lateral motion devices and much play on the outer axles. To limit this problem, the driving wheels were generally small, up to 64 inches (1,630 millimetres), which in turn generated the problem of insufficient counterweights to balance the weight of the driving rods.
The 2-10-2's inherent problem was the low speed restriction on the type, which was about 35 miles per hour (56 kilometres per hour). Further, the 2-10-2 had other inherent restrictions. The massive cylinders that were required on locomotives in the United States for high tractive effort had the result that no reasonably sized valves could admit and exhaust steam at a sufficient rate to permit fast running. In addition the 2-10-2, like the 2-6-2, had its main rod connected to the third axle, very near to the centre of gravity, which created a violent nosing action when operating at speed. The peak of the 2-10-2 design limitations was reached in the Unites States in 1926, and was overcome with the advent of the superior 2-10-4 design.
Locomotives with a 2-10-2 wheel arrangement were used in a number of countries around the world, including those in North America, Western Europe, China, the Soviet Union and Africa. Continental Europe saw a fair number of 2-10-2s, although the type was always less popular than 2-8-2 Mikados and 2-10-0 Decapods. A large number of European 2-10-2s were tank locomotives, taking advantage of the symmetrical nature of the wheel arrangement.
Belgian Congo 
Two classes of 2-10-2 locomotives were used in the Belgian Congo.
- Two locomotives were built by Du Haine Saint-Pierre for the CF du Congo Superieur aux Grands Lacs Africains in 1937, numbered 60 and 61. They had 510 by 530 millimetres (20 by 21 inches) cylinders and 1,060 millimetres (42 inches) diameter driving wheels, with a working order mass of 64.7 tonnes (63.7 long tons; 71.3 short tons).
- One locomotive was built for the CF du Bas-Congo au Katanga by Société Anonyme John Cockerill in 1947, numbered 901 and later renumbered 802. It had 540 by 550 millimetres (21 by 22 inches) cylinders and 1,100 millimetres (43 inches) diameter driving wheels, with a working order mass of 94.1 tonnes (92.6 long tons; 103.7 short tons), a grate area of 4 square metres (43 square feet) and a tractive effort at 65% boiler pressure of 14,215 kilograms-force (139,400 newtons; 31,340 pounds-force).
The mainstay of Chinese steam was their 2-10-2 locomotives. This was the wheel arrangement of the Chinese QJ class locomotives that were based on the Soviet LV class. They were built until 1988 and were still in widespread service until the final steam runs in 2003.
After retirement some of these QJ class 2-10-2 steam locomotives found their way to the United States of America, where they are used in excursion service. In 2006 Multipower International restored two Chinese QJ class locomotives to Federal Railroad Administration (FRA) Part 230 specifications and delivered them to Railroad Development Corporation.
Examples on the German railway systems included classes BR84 and BR85, both standard designs built in 1935 and 1937 respectively, and class BR95, built in 1922 by the Prussian State Railways. From 1936 the German railways built 28 3-cylinder 2-10-2 tender freight locomotives of class BR45, which were the largest steam locomotives on the system.
While the 2-10-2 wheel arrangement was not very common in Africa, the Lourenco Marques system in Mozambique (Caminhos de Ferro de Moçambique or CFM) had altogether thirty-seven locomotives of this type in three classes.
- Nine locomotives of the Series 200, numbered 201 to 209, were built by Baldwin Locomotive Works between 1915 and 1919.
- Six more Santa Fe type locomotives of the Series 214, numbered 214 to 219, were built by Henschel and Son in 1951.
- Twenty-two locomotives of the Series 250, numbered 251 to 272, were built by Henschel in 1955.
Romania designed its 151.000 Class as freight locomotives, in order to serve the Căile Ferate Române (CFR). These locomotives used a straightforward two-cylinder 650 by 720 millimetres (25.591 by 28.346 inches) engine with 1,500 millimetres (59 inches) diameter driving wheels and a total weight in working order of 123 tonnes (121 long tons; 136 short tons). The heating surface of the boiler was 254.8 square metres (2,743 square feet), of which 98.5 square metres (1,060 square feet) were superheated, while the grate area was 4.72 square metres (50.8 square feet). At 21,294 kilograms-force (208,820 newtons; 46,950 pounds-force) tractive effort, they were the most powerful steam locomotives built in Romania.
Two of these locomotives were built by the Malaxa Works in 1939 and 1941, numbered 151.001 and 151.002. Number 151.002 was preserved.
South Africa 
On 3 ft 6 in (1,067 mm) Cape gauge, this wheel arrangement was first used by the South African Railways (SAR) in 1927. Two Class 18 steam locomotives, the most powerful non-articulated locomotives to see service on the SAR, were introduced on the line between Witbank and Germiston in an attempt to ease problems that were being experienced with increasingly heavy coal trains. It was designed by Colonel F.R. Collins, Chief Mechanical Engineer of the SAR from 1922 to 1929, and built by Henschel and Son in Germany. They were three cylinder locomotives, with the two outer cylinders using Walschaerts valve gear and the inner cylinder using Gresley conjugated valve gear that was actuated by the motions of the outer cylinders.
One more 2-10-2 locomotive, the Class 20, was designed by A.G. Watson, Chief Mechanical Engineer from 1929 to 1936. Only one locomotive was built by the SAR at its Pretoria Mechanical Shops at Salvokop in 1935.
In 1950 this sole Class 20 locomotive was modified to an experimental condensing locomotive, equipped with a condensing tender that was ordered from Henschel in Germany in 1948. Beginning in 1951, tests with the condensing Class 20 were conducted in the Eastern Transvaal and the Karoo. The positive results of the condensing trials proved the viability of condensing locomotives in South Africa and led to the introduction of the Class 25 4-8-4 condensing locomotive fleet in 1953.
Soviet Union 
In the Soviet Union 2-10-2 type locomotives were used to haul heavy freight trains. Two series were relatively common, the FD (for Felix Dzerzhinsky) with more than three thousand built through the 1930s, and the LV (Lebedyanskii, modified by the Voroshilovgrad factory).
The FD class was developed from ALCO and Baldwin heavy freight locomotives that were imported to Soviet Russia, where they were designated as the Ta and Tb classes respectively. The first FD class locomotive was built at the Lugansk Locomotive Factory in 1931.
In 1932 the Voroshilovgradskom plant started with the mass production of ФД20 locomotives. In the process of production their construction was improved constantly. At the outbreak of the Great Patriotic war in 1941 production was interrupted, and was only resumed in 1942 when four locomotives were built in Ulan Ude. The total production was 2927 locomotives of ФД20, and 286 locomotives of ФД21. The two subclasses only differed in the type of superheater.
In 1958 1,054 FD class locomotives were sold to China, where they worked until the 1980s. A much lesser number were sold to North Korea at around the same time.
The LV class was developed from the previous L class 2-10-0 locomotive by the Voroshilovgrad factory. It used a feedwater heater to increase thermal efficiency and it was the most efficient freight steam locomotive in the Soviet Union, with thermal efficiency of 9.3%. The first prototype was named OR18-01 (October Revolution factory, 18 tonne axle load). A total of 522 LV class locomotives were built. Several were preserved, including the first, OR18-01, and the last, LV-0522.
In Spain, the 2-10-2 wheel arrangement was represented by one series of twenty-two locomotives. They were initially ordered for the Compañía del Norte, but the RENFE kept the entire series in reserve. Built between 1941 and 1944 in the La Maquinista Terrestre y Maritima SA factory in Barcelona for hauling heavy coal trains, they were amongst the most powerful steam locomotives in Europe. They had three cylinders, but used simple expansion and were known as "Santa Fe" locomotives.
United States of America 
In the United States the 2-10-2 type was produced between 1914 and 1930, after which its limitations became apparent and larger locomotives were built. About 2,200 were built, including about 500 of the two United States Railroad Administration (USRA) World War I standard designs. There were two USRA standard forms of the 2-10-2, the heavy locomotive and a light version.
The heaviest 2-10-2s were the ten built by Baldwin Locomotive Works for the Reading Railway circa 1931, weighing 451,000 pounds (205,000 kilograms) or 225.5 short tons (201.3 long tons; 204.6 tonnes), engine only.
At 104,000 pounds-force (460 kilonewtons) the Illinois Central Railroad’s 2800-class rebuilds probably had the highest calculated tractive effort of any two-cylinder steam locomotive, even though the weight on drivers was only 333,000 pounds (151,050 kilograms) or 166.5 short tons (148.7 long tons; 151.0 tonnes).
The Baltimore and Ohio Railroad (B&O) placed its first order for a 2-10-2 with Baldwin in 1914. It was numbered 6000. From 1914 to 1956 these B&O 2-10-2 engines bore numbers commencing with 6, hence their nickname "Big Sixes". Designated the S class, there were several variant sub-classes. The engines were heavy and powerful and they held off the diesel-electrics until 1951, when they gradually began to be withdrawn. They were all scrapped by 1959.
- Swengel, Frank, M. (1967). The American Steam Locomotive, Vol. 1. The Evolution of the Steam Locomotive. MidWest Rail Publications.pp.92, 138, 148-149, 172-173, 192-193
- Blanchart, De Deurwaerder, Nève, Robeyns & Van Bost (1999). Le Rail au Congo Belge, Tome II, 1920-1945. Brussells: G Blanchart & Cie. pp 340-341, 353, 420. ISBN 2-87202-015-2.
- Blanchart, De Deurwaerder, Nève, Robeyns & Van Bost (2008). Le Rail au Congo Belge, Tome III, 1945-1960. Brussells: Editions Masoin. pp 154, 164-165, 418. ISBN 2-9600471-0-9.
- Multipower International, Inc. - QJ Gallery
- Rolling Stock Diagrams, Caminhos de Ferro de Moçambique - Divisão de L. Marques
- Henschel-Lieferliste (Henschel & Son works list), compiled by Dietmar Stresow
- 151.000 Class Locomotive at railwayfan.ro (Romanian)
- Holland, D.F. (1972). Steam Locomotives of the South African Railways, Volume 2: 1910-1955 (1st ed.). Newton Abbott, Devon: David & Charles. pp. 51–52, 71, 107. ISBN 0715354272.
- Paxton, Leith; Bourne, David (1985). Locomotives of the South African Railways (1st ed.). Cape Town: Struik. pp. 10–11, 69, 73–74. ISBN 0869772112.
- Durrant, A E (1989). Twilight of South African Steam (1st ed.). Newton Abbott, London: David & Charles. pp. 27–30, 63–64. ISBN 0715386387.
- South African Railways & Harbours Photo Journal, Vol. 1, no 8, pp. 1-3, by Les Pivnic
- Russian article on the 2-10-2
- Railography : Chinese Steam Profiles - FD Class 2-10-2
- Staufer, Alvin F. (ed.), B&O Power: Steam, Diesel and Electric Power of the Baltimore and Ohio Railroad 1829-1965, Staufer, Medina, n.d. pp. 152-167