Under the Whyte notation for the classification of steam locomotives, a 4-8-2 locomotive has four leading wheels (generally arranged in a leading truck), eight coupled driving wheels and two trailing wheels (often but not always in a trailing truck).
This locomotive type, an obvious progression from the 4-6-2 "Pacific" type, was first built by ALCO for the Chesapeake and Ohio Railroad in 1911. ALCO combined the traction of the eight-drivered 2-8-2 "Mikado" with the excellent tracking qualities of the Pacific's four-wheel leading truck. The C&O named this locomotive type the "Mountain" after the Allegheny Mountains the type was built to tackle.
Although the C&O intended their new Mountains for passenger service, the type proved ideal for the new, faster freight service the railroads were introducing. Many 4-8-2s were built for dual service.
Mountain type locomotives were built for 41 American railroads; approximately 2200 were built. The largest users were the New York Central Railroad with 435 of what they named the Mohawk type (the Water Level Route had no need for "Mountains", after all!); the Pennsylvania Railroad with 224 class M1 and M1a locomotives, used mostly for fast freight service; the Florida East Coast with 90 passenger locomotives; the New Haven with 70; and the Southern Railroad with 58.
Outside of the United States locomotives of this type were rarer. Britain's population of Mountains consists entirely of two 15 inch gauge locomotives on the Romney, Hythe and Dymchurch Railway, though a number of double-Mountain (4-8-2+2-8-4) Garratts were built for export. In France, these locomotives (known as the 241 type) were rather more common. The Czech railways also employed a design of 4-8-2.