A booster engine is a component that can be added to a steam locomotive to drive the trailing wheels. Most steam locomotives have one or more sets of wheels that only exist to provide additional stability at high speeds. These wheels can be used for additional traction and power when the locomotive first starts moving if a booster engine is added. The booster engine typically consists of a steam engine with two cylinders that is driven by the same steam as the rest of the locomotive. After the locomotive reaches a particular speed, booster engines are typically designed to disengage so that the trailing wheels spin freely.
Booster engines were not used universally around the world, as there was some disagreement about whether the additional cost was justified. These devices were most widely used in the United States and Australia, though there were many lines in those countries that did not use them either. Several lines in Canada and New Zealand also used booster engines. In many cases, a booster was intended to provide extra power on steep or mountainous lines.
In most cases, the booster engine would be installed in the undercarriage of a locomotive behind the trailing wheels. These devices were installed after the construction of the locomotive, so they tended to add both extra cost and complexity. In order to operate they required access to steam from the boiler and a mechanical connection to a trailing axle.
The steam supply for a booster engine could be fairly complex, including a variety of check valves, cut-off valves, and chokes. Boosters typically had their own dedicated pressure gauge and lubrication system, which could also increase the complexity of the system. In order to connect the booster engine to the axle, either one or two rocker idlers were used. One of these devices would allow the booster to operate in a forward direction, and a second would be used if it was a reversible unit. Additional mechanisms could automatically disengage the booster after a particular speed was reached.
In some locomotives that lacked trailing wheels, a booster would be installed on the front wheels of the tender. This configuration was also sometimes used on switching locomotives, in which case the drive wheels of the tender could be connected to the other wheels with coupling rods. Short coupling rods such as these restricted speed, which was not a concern with switching locomotives that were confined to train yards.