Under the Whyte notation for the classification of steam locomotives by wheel arrangement, 6-8-6 represents the arrangement of six (6) unpowered leading wheels arranged into a three-axle leading truck, eight (8) powered driving wheels, and six (6) 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.