Tank and tender locomotives
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Steam engines must have somewhere to store their fuel and water, and the two basic ways of doing that is either aboard or in a separate wagon usually coupled behind the cab, a tender. The former type is called a tank locomotive and the latter a tender locomotive. The fuel, when coal or wood, must be accessible to the fireman and is therefore usually stored immediately behind or (on tank locomotives) in front of the cab.
A third, rarer, basic type is the tank tender engine, combining on-board storage with a tender. On the Ffestiniog, however, it's a common type, represented by the England Engines and the "ladies" Blanche and Linda.
Tank locomotive types
As for the water tanks, a tank locomotive with those under the boiler, between the frames, is a well tank; flanking the boiler, a side tank; and straddling the boiler, a saddle tank locomotive.
Of articulated locomotives, types like the Fairlie and Garratt are tank locomotives, but seldom so called because that arrangement follows from the type. Others, like the Mallet or the Meyer and Kitson types, can be either.
Except for the Garratt and a few other mainly articulated examples, the large mainline locomotives have usually been of the tender type, having more room for both boiler and supplies. Tank locomotives have been largely confined to shunting, short hauls and secondary and industrial lines, where compactness and good reverse running capability have been more important.