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In order that a clear understanding may be had of the various types of locomotives, a classification is given according to wheel arrangement. In the Whyte system of classification, which is quite largely used, each set of trucks and driving wheels is grouped by number beginning at the pilot or front end of the engine. Thus, 260 means a Mogul, and 460, a 10-wheel engine. The first figure, 2, in 260 denotes that a 2-wheeled truck is used in front; the figure 6, that there are six coupled drivers, three on each side; and the 0, that no trailing truck is used. This scheme gives both a convenient and easy method of classifying locomotives.

In Table II is given the classification of the locomotives used on American railroads.

The method may be further extended to include the weights of locomotives. The total weight is expressed in units of 1,000 pounds. Thus: A Pacific locomotive weighing 189,000 pounds would be classified as Type 462-189. If the locomotive is a compound, a letter C would be used instead of the dash. Thus: Type 462-C-189. If tanks are used instead of a separate tender, the letter T would be substituted for the dash. Thus: A tank locomotive having four driving wheels, a 4-wheel leading truck, and a 4-wheel rear truck, weighing 114,000 pounds would be classified as Type 444-T-114.

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