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|Classification of Steam Locomotives.|
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.
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