Tank locomotives carried their coal and water in integral coal bunkers and water tanks instead of a separate tender. This not only reduced the capital cost of a locomotive, but also made its operation more convenient and cheaper. Where there were runs of about 20 miles or shunting work or branch lines, these locomotives were never very far from obtaining water, so there was little point in hauling a tender full of water.
Tank locomotives carry a supply of coal behind the cab in a bunker that often covers a small supplementary water tank. A greater quantity of water is carried in tanks that flank the boiler in the three positions as shown above. Because of their short length and all-round visibility, these engines were popular for yard work, especially since as much of their time is in backward motion as well as forward. For work in docks and the restricted areas of factory sidings where turntables were not available, they were an indispensable choice of engine
The origin of the tank locomotive is uncertain. In Warren's A Century of Locomotive Building there is a reference to a letter written by John B. Jervis, Chief Engineer of the Delaware and Hudson Canal Company in America, and dated the 16th of January 1828, "It is desirable to dispense with the tender carriage, to have a water tank fixed to the engine carriage . . . " Whether such an engine was ever built is not known but the first (according to Ahrons) evidence of a tank locomotive was one built by Dr. Church of Birmingham in 1837.
A popular type of design was the side tank as shown above left. These consisted of two vertical, narrow tanks that rested on the running plate on each side of the boiler. Water capacity varied in Great Western engines from 800 gallons in the 1400 and 5800 class 0 - 4 - 2T's up to 2500 gallons for the 7200 class 2 - 8 - 2T's.
Another design was the saddle tank as shown above centre. The length of these tanks could cover the whole of the boiler and smokebox or just the boiler. An example of the saddle tank is the Great Western 1361 0 - 6 - 0ST class (ST denoting saddle tank). These water capacity of these particular engines was 800 gallons.
As a modification of the saddle tank, pannier tanks locomotives were introduced by the Great Western to permit the use of a Belpaire boiler. This is shown above right. There are several examples of GWR pannier tanks, but the most popular is the 5700 0 - 6 - 0PT class which had a tank capacity of 1200 gallons.
Another example although not shown was the well tank. The water was carried in tanks fitted between the frames of the locomotive, however this form of engine never found favour in Britain but was widely used in Continental Europe usually in conjunction with smaller side tanks.
Tanks were usually divided by baffles to prevent water surges. The express tank was especially subject to this - the disastrous 1927 derailment at Sevenoaks was thought to result from high speed and a bumpy track giving rise to the water in the tanks to surge.
With short distances and high population density, the popularity of the tank locomotive was high in Britain. For example, the Great Western at the time of Grouping in 1923 had 3944 locomotives, 2442 of which were tanks, however US railroads had a few for commuter trains but shunting was entrusted to six- and eight-wheeler tender locomotives.