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.
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.
It's important to stress that wheels, not axles, are what is counted in this system. Other classification schemes in use elsewhere (such as in France) count axles.
The system had to be extended with the advent of articulated locomotives. The scheme generally adopted is that locomotives such as Garrattss, where there are, in effect, two seperate 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 Malletss, 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.
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.
|ooOOoo||4-4-4||Reading, Jubilee (Canada)|
|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|
|ooOO OOOoo||4-4-6-4||(PRR Q2)|
|ooOOO OOoo||4-6-4-4||(PRR Q1)|
|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|
Garratts are almost always two identical locomotive frames back-to-back, and are thus called Double Pacifics, Double Northerns etc.