Overview
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.
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