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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. Whyte's system counts the number of leading wheels, then the number of driving wheels, and finally the number of trailing wheels, this being the common pattern of the conventional steam locomotive.
It's important to stress that wheels, not axles, are what is counted in this system. Other classification schemes in use elsewhere (like the UIC classification) count axles.
The system had to be extended with the advent of articulated locomotives. The scheme generally adopted is that locomotives such as Garratts, where there are, in effect, two separate locomotives joined by a common boiler, are classified by using a plus sign in 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 in between powered axles, are just written by adding extra numbers in the middle; each number represents a grouping of wheels. Thus a Big Boy is written under this modified Whyte notation as 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.
In addition the suffix 'T' is often used to indicate a tank locomotive (otherwise, a tender locomotive is assumed). In British practice, this is sometimes extended to indicate what type of tank locomotive. When this is done, a plain 'T' means side tank, 'ST' means saddle tank, PT means pannier tank and WT stands for well tank. Where a 'T' suffix is followed by '+T', this indicates a tank locomotive that has a tender for additional coal or water capacity.
The limitations of the Whyte system for 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.
Here is a list of the most common wheel arrangements: in the illustration, which should be read left to right, with the front of the locomotive to the left, small o is a carrying axle, and a big O is a driving axle.
|oOo||2-2-2||Single, Jenny Lind|
|ooOOoo||4-4-4||Reading, Jubilee (Canada)|
|ooOOO||4-6-0||Ten-Wheeler (not Britain)|
|oOOOOo||2-8-2||Mikado, Mike, MacArthur|
|ooOOOOoo||4-8-4||Northern, Niagara, Confederation, Dixie, Greenbrier, Pocono, Potomac|
|oooOOOOooo||6-8-6||(Used only by the Pennsylvania Railroad's steam turbine locomotive)|
|OOOOO||0-10-0||Ten-Coupled, (rarely) Decapod|
|oOOOOOoo||2-10-4||Texas, Selkirk (Canada)|
|ooOOOOOo||4-10-2||Southern Pacific, Overland|
|ooOOOOOOOoo||4-14-4||(Only one engine ever used this arrangement, the Soviet AA20. It was a failure.)|
|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|
|oOOOO-OOOOoo||2-8-8-4||Yellowstone (and, running in reverse, SP Cab Forward)|
|ooOOOO-OOOOo||4-8-8-2||(Southern Pacific cab forward )|
|oOOOOO-OOOOOo||2-10-10-2||(Santa Fe and Virginian RR's)|
|oOOOO-OOOO-OOOO-o||2-8-8-8-2||Triplex (Erie RR)|
Garratts are almost always two identical locomotive frames back-to-back, and are thus called Double Pacifics, Double Northerns etc.
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