The Whyte notation for classifying steam locomotives by wheel arrangement was devised by Frederick Methvan Whyte and came into use in the early 20th century encouraged by an editorial in American Engineer & Railroad Journal (Editorial December 1900). Whyte's system counts the number of leading wheels, then the number of driving wheels, and finally the number of trailing wheels, groups of numbers being separated by dashes. Other classification schemes, like UIC classification and the French system for steam locomotives, count axles rather than wheels.
Articulated locomotives such as Garratts, which are effectively two locomotives joined by a common boiler, have a + between the arrangements of each engine. Thus, a 'double Pacific' type Garratt is a 4-6-2+2-6-4.
Simpler articulated types such as Mallets, where there are no unpowered axles between powered axles, have extra groups of numbers in the middle. Thus a Big Boy is a 4-8-8-4; there are two leading axles, one group of four driving axles, another group of four driving axles, and then two trailing axles.
The suffix T indicates a tank locomotive; otherwise, a tender locomotive is assumed. In British practice, this is sometimes extended to indicate the type of tank locomotive: T means side tank, PT pannier tank, ST saddle tank, WT well tank. T+T means a tank locomotive that has a tender for additional coal or water capacity.
Internal combustion locomotives
In Britain, small diesel and petrol locomotives are usually classified in the same way as steam locomotives, e.g. 0-4-0, 0-6-0, 0-8-0. This may be followed by D for diesel, P petrol, and another letter describing the transmission: E for electric, H hydraulic, M mechanical. Thus 0-6-0DE denotes a six-wheel diesel locomotive with electric transmission. Where the axles are coupled by chains or shafts (rather than side-rods), or are individually driven, the terms 4w, 6w or 8w are generally used. Thus 4wPE indicates a four-wheel petrol locomotive with electric transmission. For large diesel locomotives the UIC classification is used.
The limitations of the Whyte system in classifying locomotives that did not fit the standard steam locomotive pattern led to the design of other forms of classification. Most commonly used in Europe is the UIC classification scheme, based on German practice, which can more completely define the exact layout of a locomotive.
In American (and to a lesser extent British) practice, most wheel arrangements in common use were given names, often from the name of the first such locomotive built. (For example, the 2-2-0 is named Planet.) (This naming convention is reminiscent of the naming of warship classes.)
Common wheel arrangements
Here is a list of the most common wheel arrangements: in the illustration the front of the locomotive is to the left, o is a carrying axle, O a driving axle.
|oOo||2-2-2||Single, Jenny Lind|
|ooOOoo||4-4-4||Reading, Jubilee (Canada)|
|OOO||0-3-0||(one driving wheel per axle; used on Patiala State monorail and also on the Listowel and Ballybunion Railway)|
|OOO||0-6-0||Six-Coupled, Bourbonnais (France), USRA 0-6-0 (United States)|
|ooOOO||4-6-0||Ten-Wheeler (not Britain)|
|OOOO||0-8-0||Eight-Coupled, USRA 0-8-0 (United States)|
|oOOOOo||2-8-2||Mikado, Mike, MacArthur|
|oOOOOooo||2-8-6||Used only on four Mason Bogie locomotives|
|ooOOOOoo||4-8-4||Northern, Niagara, Confederation, Dixie, Greenbrier, Pocono, Potomac, Golden State, Western, General, Governor, Big Apple, GS Series "Daylight" (Southern Pacific)|
|ooOOOOooo||4-8-6||Proposed by Lima, never built|
|oooOOOOooo||6-8-6||(PRR S2 steam turbine locomotive)|
|OOOOO||0-10-0||Ten-Coupled, (rarely) Decapod|
|oOOOOO||2-10-0||Decapod, Russian Decapod|
|oOOOOOo||2-10-2||Santa Fe, Central, Decapod (only on the Southern Pacific)|
|oOOOOOoo||2-10-4||Texas, Colorado (CB&Q), Selkirk (Canada)|
|ooOOOOO||4-10-0||Mastodon, Gobernador (in honor of El Gobernador)|
|ooOOOOOo||4-10-2||Southern Pacific, Overland|
|ooOO OOoo||4-4-4-4||(PRR T1)|
|oooOO OOooo||6-4-4-6||(PRR S1)|
|ooOO OOOoo||4-4-6-4||(PRR Q2)|
|ooOOO OOoo||4-6-4-4||(PRR Q1)|
|Mallet and Simple Articulated Locomotives|
|oOOO-OOO||2-6-6-0||Denver & Salt Lake|
|oOOO-OOOoo||2-6-6-4||Norfolk & Western|
|oOOO-OOOooo||2-6-6-6||Allegheny, Blue Ridge|
|ooOOO-OOOo||4-6-6-2||(Southern Pacific class MM-2)|
|oOOO-OOOO||2-6-8-0||(Southern Railway, Great Northern Railway)|
|ooOOOO-OOOOo||4-8-8-2||Southern Pacific cab forward classes AC-4 through AC-12 (except AC-9)|
|oOOOOO-OOOOOo||2-10-10-2||(Santa Fe and Virginian railroads)|
|oOOOO-OOOO-OOOO-o||2-8-8-8-2||Triplex (Erie RR)|
- Colvin, Fred H. (1906). The railroad pocket-book: a quick reference cyclopedia of railroad information. New York, Derry-Collard; London, Locomotive Publishing Company (US-UK co-edition), page L-9.
has media related to:
- AAR wheel arrangement
- Wheel arrangement
- UIC classification
- Category:Locomotives by wheel arrangement
|Cab positioning (Short hood / Long hood)||Cab forward • Sharknose • Steeplecab • Cab unit • Hood units • Boxcab • Long hood forward|
|Wheel arrangement||AAR wheel arrangement • UIC classification • Whyte notation|