Steam Locomotive - Categorisation

Categorisation

Steam locomotives are categorised by their wheel arrangement. The two dominant systems for this are the Whyte notation and UIC classification.

The Whyte notation, used in most English speaking and Commonwealth countries, represents each set of wheels with a number. These numbers typically represented the number of un-powered leading wheels, followed by the number of driving wheels (sometimes in several groups), followed by the number of un-powered trailing wheels. For example, a yard engine with only 4 drive wheels would be shown as a "0-4-0" wheel arrangement. A locomotive with a 4 wheel leading truck, followed by 6 drive wheels, and a 2 wheel trailing truck, would be classed as a "4-6-2". Different arrangements were given names which usually reflect the first usage of the arrangement; for instance the "Santa Fe" type (2-10-2) is so called because the first examples were built for the Atchison, Topeka and Santa Fe Railway. These names were informally given and varied according to region and even politics.

The UIC classification is used mostly in European countries apart from the United Kingdom. It designates consecutive pairs of wheels (informally "axles") with a number for non-driving wheels and a capital letter for driving wheels (A=1, B=2, etc.) So a Whyte 4-6-2 designation would be an equivalent to a 2-C-1 UIC designation.

On many railroads, locomotives were organised into classes. These broadly represented locomotives which could be substituted for each other in service, but most commonly a class represented a single design. As a rule classes were assigned some sort of code, generally based on the wheel arrangement. Classes also commonly acquired nicknames, such as 'Pugs', representing notable (and sometimes uncomplimentary) features of the locomotives.