Whyte notation

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A selection of early 20th century locomotive types according to their Whyte notation and their comparative size
File:Locomotive classification--Colvin 1906--300dpi.jpg
Whyte notation as of 1906. From a handbook for railroad industry workers published in 1906.[1]

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.

Thus, a locomotive with two leading axles (and thus four wheels) in front, then three driving axles (six wheels) and followed by one trailing axle (two wheels) is classified as a 4-6-2.


Articulated locomotives

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.

Arrangement Whyte Classification Name
Non-Articulated Locomotives
Oo 0-2-2 Northumbrian
oO 2-2-0 Planet
oOo 2-2-2 Single, Jenny Lind
oOoo 2-2-4  
ooO 4-2-0 Jervis
ooOo 4-2-2 Bicycle
ooOoo 4-2-4  
oooO 6-2-0 Crampton
OO 0-4-0 Four-Coupled
OOo 0-4-2  
OOoo 0-4-4 Forney
oOO 2-4-0 Porter
oOOo 2-4-2 Columbia
oOOoo 2-4-4  
ooOO 4-4-0 American, Eight-wheeler
ooOOo 4-4-2 Atlantic
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)
OOOo 0-6-2  
OOOoo 0-6-4  
oOOO 2-6-0 Mogul
oOOOo 2-6-2 Prairie
oOOOoo 2-6-4 Adriatic
oOOOooo 2-6-6
ooOOO 4-6-0 Ten-Wheeler (not Britain)
ooOOOo 4-6-2 Pacific
ooOOOoo 4-6-4 Hudson, Baltic
OOOO 0-8-0 Eight-Coupled, USRA 0-8-0 (United States)
OOOOo 0-8-2  
oOOOO 2-8-0 Consolidation
oOOOOo 2-8-2 Mikado, Mike, MacArthur
oOOOOoo 2-8-4 Berkshire, Kanawha
oOOOOooo 2-8-6 Used only on four Mason Bogie locomotives
ooOOOO 4-8-0 Mastodon, Twelve-Wheeler
ooOOOOo 4-8-2 Mountain, Mohawk
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 0-10-2 Union
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
OOOOOO 0-12-0 Twelve-Coupled
oOOOOOO 2-12-0 Centipede
oOOOOOOo 2-12-2 Javanic
oOOOOOOoo 2-12-4  
ooOOOOOOo 4-12-2 Union Pacific
ooOOOOOOOoo 4-14-4 AA20
Duplex Locomotives
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
OO-OO 0-4-4-0
o-OO-OO-o 2-4-4-2  
OOO-OOO 0-6-6-0 Erie
oOOO-OOO 2-6-6-0 Denver & Salt Lake
oOOO-OOOo 2-6-6-2  
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)
ooOOO-OOOoo 4-6-6-4 Challenger
oOOO-OOOO 2-6-8-0 (Southern Railway, Great Northern Railway)
OOOO-OOOO 0-8-8-0 Angus
oOOOO-OOOO 2-8-8-0 Bull Moose
oOOOO-OOOOo 2-8-8-2 Chesapeake
oOOOO-OOOOoo 2-8-8-4 Yellowstone
ooOOOO-OOOOo 4-8-8-2 Southern Pacific cab forward classes AC-4 through AC-12 (except AC-9)
ooOOOO-OOOOoo 4-8-8-4 Big Boy
oOOOOO-OOOOOo 2-10-10-2 (Santa Fe and Virginian railroads)
oOOOO-OOOO-OOOO-o 2-8-8-8-2 Triplex (Erie RR)
oOOOO-OOOO-OOOO-oo 2-8-8-8-4 Triplex


  1. 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. 

See also

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