Under the Whyte notation for the classification of steam locomotives by wheel arrangement, 6-8-6 represents the arrangement of six unpowered leading wheels, eight powered and coupled driving wheels, and six unpowered trailing wheels.
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.[1]