thumb|[[New York Central Railroad's Empire State Express takes on water from the track pan at Palatine, New York, in 1905.]]
A water trough (British terminology), or track pan (American terminology), is a device to enable a steam locomotive to replenish its water supply while in motion. It consists of a long trough filled with water, lying between the rails. When a steam locomotive passes over the trough, a water scoop can be lowered by the fireman, and the speed of forward motion forces water into the scoop, up the scoop pipe and into the tanks or locomotive tender.
Origin
thumb|right|[[John Ramsbottom (engineer)|Ramsbottom water troughs on a four-line stretch of the West Coast Main Line, England, in 1904]]
Steam locomotives consume a considerable amount of water, and the tender or side tanks need to be replenished at intervals. Traditionally the engine water was replenished during station stops, but if it was desired to run long distances without stopping, the requirement to take water was a significant limitation. The Railway Magazine reported a development by John Ramsbottom:
Ramsbottom arranged some experiments and showed that the forward motion of a scoop in a trough of water would force water up a connected pipe and into a tank. He calculated the quasi-static head produced by the forward motion: