The Norfolk and Western M, M1 and M2 Classes were a series of 4-8-0 steam locomotives owned and operated by the Norfolk and Western Railway (N&W). The M Classes were primarily assigned to pull the N&W's mainline freight trains, but following the introduction of the railway's Y Class 2-8-8-2's, the M Classes were reassigned to short line freight service.

Under the Whyte notation for the classification of steam locomotives, 4-8-0 represents the wheel arrangement of four leading wheels on two axles, usually in a leading truck or bogie, eight powered and coupled driving wheels on four axles and no trailing wheels. In North America and in some other countries the type was usually known as the Twelve-wheeler.