Under the Whyte notation for the classification of steam locomotives, 0-12-0 represents the wheel arrangement of no leading wheels, twelve powered and coupled driving wheels on six axles, and no trailing wheels.
Other equivalent classifications are:
The first example of the 0-12-0 was the Pennsylvania, designed by Jame Milholland for the Philadelphia and Reading Railroad and built at its own shops in 1863. It weighed fifty tons and was, at the time, the heaviest steam locomotive in the world. It was intended to haul Pennsylvania coal trains.
The only others of the type, was a class of ten 0-12-0T locomotives built by Hanomag in 1922 for the Bulgarian State Railways (BDŽ). They were initially numbered 4001–4010, but were renumbered 45.01 to 45.10 in 1935–1936. They were built as two-cylinder compound locomotives, with a 15-kilogram-per-square-centimetre (1.47 MPa; 213 psi) boiler feeding a 620-by-700-millimetre (24.41 in × 27.56 in) high-pressure cylinder discharging to a 900-by-700-millimetre (35.43 in × 27.56 in), both of which were connected to the 1,340-millimetre (52.76 in) driving wheels. The locomotives weighed 101 tonnes (99 long tons; 111 short tons).