PRR S1
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The Pennsylvania Railroad's class S1 comprised a single steam locomotive of 6-4-4-6 wheel arrangement in the Whyte notation, the only locomotive of such arrangement ever constructed. The S1 was a duplex locomotive: it had two pairs of cylinders, each driving two pairs of driving wheels, but its driven wheelbase was rigid, unlike similar-looking articulated locomotive designs. The locomotive was displayed at the New York World's Fair of 1939, lettered American Railroads rather than Pennsylvania Railroad. It was hoped that the locomotive could haul 1,000 tons at 100 miles per hour, but this goal was not reached. Furthermore, the locomotive's extreme length limited its usefulness as it was incapable of negotiating curves on most lines of PRR track. No further S1s were built. PRR instead turned its attention to the T1 class of 4-4-4-4 duplex locomotives, but they also met with limited success.
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