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The single S2, #6200, in a PRR promotional image.

In Whyte notation, a 6-8-6 is a steam locomotive with:

Other equivalent classifications are:

The S2 steam turbine locomotive, built for the Pennsylvania Railroad, was the only one ever to use this 6-8-6 wheel arrangement.

The engine built in 1944, used a direct-drive steam turbine, which ensured a smooth uniform power flow (torque or tractive effort) at all speeds. As the locomotive did not use cylinders, there was no rail hammering as with reciprocating engines, so that the wheels only required counter-balances for the coupling rods. Consequently, the wheel diameter was small at 68 inches (1727 mm). The turbines drove the two middle axles via a series of reduction gears, however high pressure steam hits the blades at speeds up to 2,000 "miles per hour" [sic] which in turn generated about 6,900 horsepower (5.1 megawatts). The output exceeded all conventional steam locomotives as well as diesels rated at 6,000 hp (4,500 kW), above 40 mph (64 km/h). At speeds less than 30 mph (48 km/h) steam consumption was high, but above that speed, its steam consumption was well below normal locomotives. Turbine maintenance was a major problem and the engine only ran until 1949. Some of its impressive attributes included; boiler pressure 310 psi (2.1 MPa); grate area 120 ft² (11 m²), tractive force 70,500 lbf (314 kN); engine weight 589,920 lb (267,580 kg).(Staufer, pp. 240-242)


  • Staufer, Alvin F. (1962). Pennsy Power: Steam and Electric Locomotives of the Pennsylvania Railroad 1900-1957. Wayner Publications. ISBN 0-944513-04-2.