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 S2 was the largest direct-drive turbine locomotive design ever built. Download high resolution version (863x426, 119 KB)Pennsylvania Railroad class S2 steam turbine locomotive #6200. ... Download high resolution version (863x426, 119 KB)Pennsylvania Railroad class S2 steam turbine locomotive #6200. ... 1893 map The Pennsylvania Railroad (AAR reporting mark PRR) was an American railroad existing 1846–1968, after which it merged into Penn Central Transportation. ... A rotor of a modern steam turbine, used in a power plant A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into useful mechanical work. ... Great Western Railway No. ... Locomotive wheel arrangement is how the wheels of the locomotive are arranged by type, position, and connections. ... In Whyte notation a 6-8-6 is a steam locomotive with six unpowered leading wheels arranged into a three-axle leading truck, eight powered driving wheels, and six unpowered trailing wheels arranged into a three-axle trailing truck. ... A selection of early 20th century locomotive types according to their Whyte notation and their comparative size The Whyte notation for classifying steam locomotives by wheel arrangement was devised by Frederick Methvan Whyte and came into use in the early 20th century. ... On a steam locomotive, a leading wheel or leading axle is an unpowered wheel or axle located in front of the driving wheels. ... On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotives pistons (or turbine, in the case of a steam turbine locomotive). ... On a steam locomotive, a trailing wheel or trailing axle is an unpowered wheel or axle located behind the driving wheels. ... A coupling rod or side rod connects the driving wheels of a steam locomotive. ...

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 Belpaire firebox and 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. The Norfolk & Western Railways Class J locomotive #611, a 1950 product of the railroads own Roanoke, Virginia shops. ... A PRR N1s. ... A combustion chamber is part of an engine in which fuel is burned. ... The smokebox (outlined in red) of Soo Line 1003. ... A Feedwater heater is a power plant component used to pre-heat water delivered to the boiler. ... A T1 prototype leaves Chicagos Union Station in February 1943 with the Manhattan Limited to New York. ... The Pennsylvania Railroads 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. ...

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 prodigious, wasteful amounts of steam and fuel.

That flaw, and the by then obvious alternative of the diesel locomotive, ensured #6200 would never be duplicated. The locomotive was withdrawn from service in 1949 and scrapped in 1953. Great Western Railway No. ...