Trains and Railroads

In its simplest form, a steam locomotive consists of a firebox, a boiler, a cylinder or cylinders, and wheels, all of which are mounted on a rigid frame. The flames in the firebox heat water in the boiler to create steam. The steam is directed into a cylinder where its force is used to push a plunger attached by a connector rod or gears to the driving wheel of the engine. These connecting elements force the wheels to turn, which moves the engine along the track.

Wheels are classified as drive wheels, which provide motive power, and carrying wheels, which distribute the weight of the engine and add stability. Carrying wheels are further divided into leading wheels, i.e., those ahead of the drivers, and trailing wheels, or those behind the drivers. A common classification scheme for steam locomotives gives the number of leading wheels, the number of driving wheels, and the number of trailing wheels. The "choo-choo" style engine of the American West, for instance, would be classified as a 4-4-0: four leading wheels, four drivers, and no trailing wheels.