The Pennsylvania Railroad S2 was a 6-8-6 steam locomotive in the "Turbine" series. No. 6200 was the sole example of the 6-8-6 wheel arrangement in the Whyte notation, with a six-wheel leading truck, eight driving wheels, and a six-wheel trailing truck.
The S2 Locomotive was built by Baldwin Locomotive Works and the first (and only) locomotive, No. 6200, rolling out in 1944.
It was used was used for freight and passenger service for the Pennsylvania Railroad and proved to be powerful and capable, with reserves of power at speed and reasonable fuel economy.
The S2 used a direct-drive steam turbine; the turbine was geared to the center pair of axles with the outer two axles connected by side rods. The turbine drive was easy on the track and allowed more power at the rail. While economical at speed, the locomotive was highly uneconomical at lower speed. The turbine used less steam than conventional locomotives above 30 mph (48 km/h), but below that the locomotive used too much steam and fuel.
It was retired from service in 1949 and it was scrapped in 1953 after being stored for 4 years; all the various factors leading up to the 6200's retirement ensured that this sort of design was never repeated again.
- The large 16-wheel tender was similar to that used on the PRR's other large passenger locomotives, the T1 and S1.
- Later in its life, the 6200 was fitted with smoke lifters; it was built without smoke lifters.