Tank engines were the very first types of switching or secondary steam locomotives; to whom they were used for switching or shunting rollingstock or other locomotives, and used on branchlines or as spare units for trains.
BunkerEditOn a tank engine, or locomotive, the fuel (most often coal) is carried in a bunker the (location of which can vary). On a locomotive with a trailing carrying axle or a trailing bogie, the bunker is generally situated to the rear of the cab, but in cases where the firebox overhangs the rear driving axle, it has been common practice to situate the bunker on top of and to one side of the firebox; this concentrates the weight and stabilizes the locomotive.
There are several different types, styles, or forms of the water tanks used on a tank engine:
- Side; which are the tanks which are mounted on the sides of the boiler and extend down to the footplates. These are referred to as "T" when using the Whyte N=notation.
- Saddle; which is mounted over the boiler like a saddle. These are referred to as "ST" when using the Whyte notation.
- Pannier; which are mounted on the sides of the boiler, but don't continue down to the footplates. These are referred to as "PT" when using the Whyte notation.
- Well; Which are mounted underneath the locomotive, these are mostly used only on very small engines such as 0-4-0WT locomotives. These are referred to as "WT" when using the Whyte notation.
- Fireless; which is a large, circular tank used for housing boiled and pressurized water heated to have a tank engine travel at slow speeds around industrial areas without creating smoke and sparks from coal-powered fireboxes.
A tank "engine" is often referred to by the actual engine powering the locomotive rather than the locomotive itself.
Wheel Arrangements are often referred to by an added "T" for tenderless to avoid confusion of a steam locomotive with a tender. (Such as: 0-6-0T and 0-6-0.)
"F" and "S" are also used in wheel arrangements to distinguish "saddletank" and "fireless" tank engines.