Mallet Locomotive
Designed originally by Anatole Mallet in 1884, the design was a compound locomotive with two sets of engines on bogie frames. Later versions were simple expansion locomotives and were developed in the US to the largest locomotives ever built, the Union Pacific 4-8-8-4 'Big Boy' class. A Mallet has the boiler rigidly fixed to the rear engine unit. The front engine unit is articulated to the rear and the boiler unit is free to move over it. This can result in the boiler unit projecting outside the front engine unit on curves.
