2-10-2

2-10-2 (Santa Fe)

ATSF tandem compound 2-10-2

Equivalent classifications UIC class 1E1 French class 151 Turkish class 57 Swiss class 5/7 Russian class 1-5-1

First known tank engine version First use 1922 Country Germany Locomotive Prussian T 20 Railway Deutsche Reichsbahn Designer Prussian state railways Builder Borsig & Hanomag

First known tender engine version First use 1903 Country United States of America Locomotive AT&SF 900 class Railway Atchison, Topeka and Santa Fe Builder Atchison, Topeka and Santa Fe Evolved from 2-10-0

First known "True type" version First use 1919 Country United States of America Locomotive AT&SF 3800 class Railway Atchison, Topeka and Santa Fe Builder Atchison, Topeka and Santa Fe Evolved to 2-10-4 Benefits Larger and deeper firebox Drawbacks Nosing action at speed

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 of America 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.

The 2-10-2 wheel arrangement evolved in the United States 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 coupled wheels presented a problem on curves, requiring flangeless drivers, lateral motion devices and much sideplay on the outer axles. To limit this problem, the coupled wheels were generally small, up to 64 inches (1,630 millimetres) in diameter, 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 middle coupled 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 United States in 1926 and was overcome with the advent of the superior 2-10-4 design.