Steam Locomotives

The reciprocating steam locomotive is a self-contained power unit consisting essentially of a steam engine and a boiler with fuel and water supplies. Superheated steam, controlled by a throttle, is admitted to the cylinders by a suitable valve arrangement, the pressure on the pistons being transmitted through the main rod to the driving wheels. The driving wheels, which vary in number, are connected by side rods. Steam locomotives are usually classified under the Whyte system, that is, by the number and arrangement of the wheels; for example, an engine classified as 2–6–0 has one pair of wheels under the front truck, three pairs of coupled or driving wheels, and no wheels under the trailing truck. In some cases the truck wheels of the tender (fuel carrier) are added.