Under the Whyte notation for the classification of steam locomotive wheel arrangements, a 2-10-10-2 is a locomotive with two leading wheels, two sets of ten driving wheels, and a pair of trailing wheels.
Other equivalent classifications are:
UIC classification: 1EE1 (also known as German classification and Swiss classification)
Italian and French classification: 150+051
Turkish classification: 56+56
Swiss classification: 5/6+5/6
The equivalent UIC classification is refined to (1′E)E1′ for Mallet locomotives. All 2-10-10-2 locomotives have been articulated locomotives, Mallet locomotives in particular.
This wheel arrangement was rare. Only two classes of 2-10-10-2 locomotives have been built; the Atchison, Topeka and Santa Fe Railway's 3000 class, and the Virginian Railway's class AE.
This class of ten 2-10-10-2 locomotives were actually rebuilt from more conventional 2-10-2 Baldwin-built locomotives by the Atchison, Topeka and Santa Fe Railway in 1911 to 1912.
Although they appeared to have exceedingly long boilers, the barrel in front of the rear set of cylinders actually contained first a primitive firetube superheater for further heating the steam before use; the steam was carried forward from the boiler proper by outside steam pipes as shown in the photograph. Also in this space was a reheater for the high-pressure exhaust before it was fed to the forward low-pressure cylinders.
In front of that, there was a feedwater heater, a space where cold water from the tender could be warmed before being injected into the water proper. This worked similarly to the boiler itself; the firetubes passed through the feedwater tank. The ATSF 2-10-10-2's, #3000 class locomotives, were the largest locomotives in the world from 1911 till possibly early 1914.
The experiment was unsuccessful and the locomotives were rebuilt to 2-10-2s during 1915–1918.Road numbers: 3000–3009
Driver diameter: 57 in (1.448 m)
Weight: 616,000 lb (279,400 kg; 279.4 t)
tender weight 266,400 lb (120,800 kg; 120.8 t)
total length 122 ft (37.19 m)
overall wheelbase 108 ft 10 in (33.17 m)
Tractive effort: 111,600 lbf (496 kN)
Boiler pressure: 225 psi (1.55 MPa)
Cylinder diameter: 28 in (711 mm) high pressure, 38 in (965 mm) low pressure
Cylinder stroke: 32 in (813 mm)
These ten locomotives were built in 1918 by ALCO for the Virginian Railway. Overall width was 144 inches (3,658 mm), so they were delivered without their cabs and the front, low-pressure cylinders and were assembled after arrival. The 48-inch (1,219 mm) low-pressure cylinders (on 90-inch or 2,286-millimetre centers) were the largest on any US locomotive; the cylinders had to be inclined a few degrees to provide clearance. The boiler was also the largest diameter of any locomotive; Railway Mech Engnr says "the outside diameter of the largest course is 112 7⁄8 inches (2,867 mm)." but the drawing shows 118 1⁄2 inches (3,010 mm) diameter at the rear tube sheet.
As seen in the photograph, the tenders were small so they could use the Virginian's existing turntables.
This class were compound Mallet locomotives: as well as being articulated between the forward, swinging engine unit and the rear fixed one, they were compound locomotives. The rear, high-pressure cylinders exhausted their steam into the huge front cylinders. Like many compound locomotives, they could be operated in simple mode for starting; reduced-pressure steam could be sent straight from the boiler to the front cylinders at low speed, for maximum tractive effort.
Calculated in the usual way (assuming equal tractive effort from the two engines and mean effective pressure adding up to 0.85 times boiler pressure) the tractive effort was 135,200 lb (61,300 kg) in compound; in the US, compound Mallets were credited with 20% more tractive effort in simple mode, or 162,200 lb (73,600 kg) for the Virginian locomotives.
Unlike some other giant locomotives of the period, the immense boilers could generate enough steam to make them a success on the slow (8 mph or 13 km/h) coal trains for which they were built. They remained in service until the 1940s and could be called the ultimate drag era locomotive. No Locomotive Example of this Type survived into Preservation.