6-8-6

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

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.

The equivalent UIC classification is 3'D3'.

The S2 steam turbine locomotive built for the Pennsylvania Railroad was the only one ever to use this wheel arrangement. Wartime rationing of lightweight steel made the locomotive far heavier than originally planned, and three-axle leading and trailing trucks were needed to handle this extra weight. Though the turbine excelled when pulling trains at high speeds, fuel and water consumption at slower speeds were unacceptably high.

Steam locomotive types

Single engine types
0-2-2 | 2-2-0 | 2-2-2 | 2-2-4 | 4-2-0 | 4-2-2 | 4-2-4 | 6-2-0
0-4-0 | 0-4-2 | 0-4-4 | 2-4-0 | 2-4-2 | 2-4-4 | 4-4-0 | 4-4-2 | 4-4-4
0-6-0 | 0-6-2 | 0-6-4 | 2-6-0 | 2-6-2 | 2-6-4 | 2-6-6 | 4-6-0 | 4-6-2 | 4-6-4
0-8-0 | 0-8-2 | 0-8-4 | 2-8-0 | 2-8-2 | 2-8-4 | 2-8-6 | 4-8-0 | 4-8-2 | 4-8-4 | 4-8-6 | 6-8-6
0-10-0 | 0-10-2 | 2-10-0 | 2-10-2 | 2-10-4 | 4-10-0 | 4-10-2
0-12-0 | 2-12-0 | 2-12-2 | 2-12-4 | 4-12-2 | 4-14-4

Duplex engine types
4-4-4-4 | 6-4-4-6 | 4-4-6-4 | 4-6-4-4

Garratt (articulated) types
0-4-0+0-4-0 | 2-6-0+0-6-2 | 4-6-2+2-6-4 | 2-8-0+0-8-2 | 4-8-4+4-8-4

Mallet (articulated) types
0-4-4-0 | 0-4-4-2 | 2-4-4-2
0-6-6-0 | 2-6-6-0 | 2-6-6-2 | 2-6-6-4 | 2-6-6-6 | 2-6-8-0 | 4-6-6-2 | 4-6-6-4
0-8-8-0 | 2-8-8-0 | 2-8-8-2 | 2-8-8-4 | 4-8-8-2 | 4-8-8-4
2-10-10-2 | 2-8-8-8-2 | 2-8-8-8-4

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