A tank locomotive or tank engine is a steam locomotive that carries its own fuel and water on it, instead of pulling it behind it in a tender. The Novelty (locomotive) which ran at the Rainhill Trials in 1829 was an example of a Well Tank. However, the more common Side tank originated about 1840 and quickly became popular for industrial tasks, and later for shunting and shorter distance main line duties. There are several different types of tank locomotive dependent upon the position and style of the water tanks and fuel bunkers. Tank locomotives have advantages and disadvantages compared to traditional tender locomotives.

The first tank locomotive was the Novelty (locomotive) which ran at the Rainhill Trials in 1829, which was an example of a Well Tank. However, the more common form of Side tankdate from the 1840s; one of the first of which was supplied by George England and Co. of New Cross to the contractors building the Newhaven, Sussex branch line for the London Brighton and South Coast Railway in 1848. In spite of the early belief that such locomotives were inherently unsafe, the idea quickly caught on, particularly for industrial use and five manufacturers exhibited designs at The Great Exhibition in 1851. These were E. B. Wilson and Company, William Fairbairn & Sons, George England, Kitson Thompson and Hewitson and William Bridges Adams. By the mid 1850s tank locomotives were to be found performing a variety of main line and industrial roles, particularly those involving shorter journeys or frequent changes in direction.