The single S2, #6200, in a PRR promotional image. The Pennsylvania Railroad's class S2 was a steam turbine-driven steam locomotive. Only one prototype was built, #6200. The S2 was of a unique wheel arrangement: 6-8-6 in the Whyte notation, featuring a six-wheel leading truck, eight driving wheels, and a six-wheel trailing truck. The S2 design was a direct-drive steam turbine; the turbine drove the wheels via gearing, being geared to the center pair of axles with the outer two axles connected by side rods. This had the advantage of simplicity, but the disadvantage that the turbine could not operate at optimal speeds over the locomotive's entire speed range. The S1 was the largest direct-drive turbine locomotive design ever built.
It was originally intended for the locomotive to be a 4-8-4, but wartime restrictions on exotic steel alloys meant that the locomotive became too heavy, and six-wheel leading and trailing trucks were needed. Two turbines were fitted, one for forward travel and a smaller one for reversing at speeds up to 22 mph. A large boiler with a long combustion chamber was fitted. Draft for the fire was provided through fans in the smokebox, providing an even draft and exhausting through a unique quadruple stack. A Worthington-pattern feedwater heater was fitted for increased efficiency. Twin air pumps for train braking were fitted below the running boards beside the smokebox front, and a large radiator assembly at the nose cooled the compressed air. A very large tender, similar to that used on the PRR's other large passenger locomotives, the T1 and S1, was provided.
In service, the locomotive proved to be powerful and capable, with ample reserves of power at speed and reasonable fuel economy. The smooth turbine drive was easy on the track and allowed more power to be delivered to the rails. The major disadvantage was that, while economical at speed, the locomotive was highly uneconomical at lower speeds. The turbine was optimised to run at its ideal speed range at above about 40 mph; below that, the locomotive used prodigous, wasteful amounts of steam.
That flaw, and the by then obvious alternative of the diesel locomotive, ensured #6200 would never be duplicated. The locomotive was withdrawn from service and scrapped.
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