|Booster on C&O 2705|
Unlike diesel locomotives whose pulling capacity is measured in horsepower (a unit of power), a steam locomotive's pulling capacity was measured in tractive effort (a unit of force). Tractive effort is a measure of the pulling force that a steam locomotive could apply from a stationary position. Generally speaking, if a steam locomotive could get a train moving from rest, it could pull it any reasonable speed. The amount of tractive effort that a steam locomotive could produce was determined by the geometry of the steam locomotive. In order to increase the pulling force beyond what the geometry of the steam locomotive could produce, boosters were sometimes added to the locomotive. A booster could either be added to the leading truck of the tender or more often, to the trailing truck of the locomotive. Since boosters were only capable of operating a lower speeds, they would either limit the locomotive to switching duties or must also have the ability to be disengaged which is the case with rear locomotive truck boosters.
A tender booster is easily spotted by the uncharacteristic connecting rods on a tender truck. Boosters applied to the trailing truck of a locomotive are harder to identify by the casual observer.