6-8-6

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.

Other equivalent classifications are:

UIC classification: 3D3 (also known as German classification and Italian classification)

French classification: 343

Turkish classification: 410

Swiss classification: 4/10

The S2 steam turbine locomotive built for the Pennsylvania Railroad was the only one ever to use this 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 did not require counter-balances. 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, above 40 mph. At speeds less than 30 mph 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)

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