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 Classification of Steam Locomotives.


WheelsNotationName
Oo0-2-2Northumbrian
oO2-2-0Planet
oOo2-2-2Jenny Lind
ooO4-2-0Jervis
ooOo4-2-2Bicycle
OO0-4-0Four Coupled
OOoo0-4-4Forney
oOO2-4-0Porter
oOOo2-4-2Columbia
ooOO4-4-0Standard
ooOOo4-4-2Atlantic
ooOOoo4-4-4Jubilee
OOO0-6-0Six Coupled
oOOO2-6-0Mogul
oOOOo2-6-2Prairie
oOOOoo2-6-4Adriatic
ooOOO4-6-0Ten-Wheeler
ooOOO4-6-0Camelback
ooOOOo4-6-2Pacific
ooOOOoo4-6-4Hudson
OOOO0-8-0Switcher
oOOOO2-8-0Consolidation
oOOOOo2-8-2Mikado
oOOOOoo2-8-4Berkshire
ooOOOO4-8-012-wheeler
ooOOOOo4-8-2Mountain
ooOOOOoo4-8-4Northern
OOOOO0-10-0Switcher
OOOOOo0-10-2Union
oOOOOO2-10-0Decapod
oOOOOOo2-10-2Santa Fe
oOOOOOoo2-10-4Selkirk
ooOOOOO4-10-0Gobernador
ooOOOOOo4-10-2Overland
OOOOOO0-12-0Switcher
oOOOOOO2-12-0Centipede
ooOOOOOOo4-12-2Union Pacific
OOO+OOO0-6-6-0Erie
oOOO+OOOooo2-6-6-6Allegheny
ooOOO+OOOo4-6-6-2Cab Forward
ooOOO+OOOoo4-6-6-4Challenger
oOOOO+OOOOo2-8-8-2Mallet
oOOOO+OOOOoo2-8-8-4Yellowstone
ooOOOO+OOOOoo4-8-8-4Big-Boy
oOOOOO+OOOOOo2·10·10·2Virginian

Different systems for denoting steam locomotive wheel arrangements were developed in different countries. In North America and the United Kingdom it was usual to refer to a steam locomotive type by its wheels rather than its axles. A wheel arrangement is described numerically by the number of wheels beginning from the front:  The number of leading wheels, followed by the number of driving wheels, followed by the number of trailing wheels, in that order. This system was invented by Frederic M. Whyte in America in 1900. A "T" at the end of a description ( e.g. 0-6-0T ) indicates a tank engine, a steam locomotive having tanks usually astride the boiler and without a tender. The table above shows how the Whyte classification system works.
 
Some European railways used a modified form of the Whyte system where the number of axles was used instead of the number of wheels, 4-6-2 becoming 231. This was further developed by the French who used numbers for non-driven axles and letters for driven axles thus, 2C1. This was re-arranged by British locomotive designer Bullied who placed the non-driven axles first in the order, then the driven axles, thus 21C. From the French system it can be seen how a non-steam locomotive axle notation system was developed.
 
1998—2001 Railway Technical Web Pages and Wikipedia.