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.
