A pannier tank is a type of locomotive which carries fuel and water in panniers along the side of the boiler. This approach to the tank locomotive is associated most particularly with the Great Western Railway (GWR), although examples could be seen in service on other railways as well. Although steam locomotives have been largely replaced by locomotives of other types, a few can be seen in action today in various corners of the world, including pannier tank locomotives.
Steam locomotives derive their power from steam. Historically, each locomotive carried a tender with fuel and water. Over time, railways began adopting tank locomotives; engines which carried fuel and water right on the locomotive, instead of in a separate tender. There were a number of advantages to this design which caused it to catch on in many corners of the railroading world, and a number of variations on the tank engine were created to meet various needs.
A locomotive with pannier tanks has a low center of gravity, which can make it safer at high speeds. The fuel and water are also readily accessible, another distinct advantage, while the bottom of the tanks is higher than that seen in side tanks, another tank style which can be used with locomotives. The design causes the locomotive to look a bit bulgy, as the tanks stick out from the side of the boiler, but some people find the look of pannier tanks rather cheerful; depending on the style, the engine can be reminiscent of a squirrel with cheeks full of nuts.
The traveling distance of a steam locomotive is limited in part by how much fuel and water it can carry, in addition to how efficiently it can use its supplies. While trains can stop to reload their tanks, it is much more convenient and efficient to be able to go straight through. The pannier tank design was intended to maximize the amount of material which could be carried without compromising the operation of the boiler.
The Great Western Railway produced pannier tank locomotives in part by modifying existing locomotives, and eventually developed several models which were built with pannier tanks installed. Enthusiasts can point to several different models and modifications used by the GWR with pannier tanks, along with examples of the pannier tank design which were once used by other railways. People who are interested in seeing this design in person may want to ask heritage railways if they use a pannier tank locomotive or know of a railway which does.