Front of locomotive to the left
The single S2, Template:Abbr 6200, in a PRR promotional image.
Under the Whyte notation for the classification of steam locomotives by wheel arrangement, 6-8-6 represents the arrangement of 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 only known example of the 6-8-6 wheel arrangement is the experimental Pennsylvania Railroad S2 steam turbine locomotive which was built in 1944 to pull passenger trains on the Pennsylvania Railroad between Chicago Illinois and Crestline Ohio until it was retired in 1949 and scrapped in 1952 . This locomotive was the only steam turbine locomotive built with a 6-8-6 locomotive wheel base just like Pennsylvania No. 6100 was the only conventional streamlined duplex steam locomotive to built in 1939 with a 6-4-4-6 locomotive wheel base making both the 6100 and 6200 the only 6-4-4-6 and 6-8-6 type locomotives to be built before and during World War 2 .