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]