A steam locomotive is normally identified by the arrangement of its wheels. This locomotive generally has two types of wheels.
Driving wheels These wheels are coupled with each other and are directly connected with the pistons. The higher the diameter of the driving wheels, the greater the tractive power, but the lower the speed.
Idle wheels These wheels distribute the load of the locomotive on the track.
A locomotive is normally designated by a three-digit code. The first digit indicates the number of idle wheels in the front, the middle digit indicates the number of driving wheels, and the last digit denotes the number of idle wheels that come after the driving wheels. A locomotive with a nomenclature of 4-6-2 indicates four idle wheels in the front, six driving wheels, and two idle wheels after the driving wheels (Fig. 24.1).
It may be noted here that the middle digit indicating the driving or coupled wheels is the most important of all. The hauling capacity of the locomotive is decided by the number of driving wheels.