A tank locomotive or tank engine is a steam locomotive that carries its water in one or more on-board water tanks, instead of a more traditional ...
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tender. Most tank engines also have bunkers (or fuel tanks) to hold fuel; in a tender-tank locomotive a tender holds some or all of the fuel, and may hold some water also.
There are several different types of tank locomotive, distinguished by the position and style of the water tanks and fuel bunkers. The most common type has tanks mounted either side of the boiler. This type originated about 1840 and quickly became popular for industrial tasks, and later for shunting and shorter-distance main line duties.
