Editing 2-10-2 (section)

==Overview==
The {{nowrap|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 [[tandem compound locomotive]]s, 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.<ref>{{Book-Van Riemsdijk-Compound Locomotives|page=40}}</ref> These first {{nowrap|2-10-2}} locomotives became the forerunners to the entire {{nowrap|2-10-2}} family.<ref name="Swengel"/>

The trailing truck allows a larger, deeper [[Firebox (steam engine)|firebox]] than that of a {{nowrap|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 device]]s and much sideplay on the outer axles. To limit this problem, the coupled wheels were generally small, up to {{convert|64|in|mm|sigfig=3}} in diameter, which in turn generated the problem of insufficient counterweights to balance the weight of the driving rods.<ref name="Swengel">{{cite book |author =Swengel, Frank M. |title=The American Steam Locomotive, Vol. 1. The Evolution of the Steam Locomotive |publisher=MidWest Rail Publications |year=1967}}pp.92, 138, 148-149, 172-173, 192-193</ref>

The {{nowrap|2-10-2}}'s inherent problem was the low speed restriction on the type, which was about {{cvt|35|mph|km/h}}. Further, the {{nowrap|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 {{nowrap|2-10-2}}, like the [[2-6-2|{{nowrap|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 (waddling) action when operating at speed. The peak of the {{nowrap|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.<ref name="Swengel"/>