The Mallet is a type of articulated locomotive invented by Anatole Mallet, a Swiss engineer. In the Mallet design, there are two powered driver trucks. The rear drivers are rigidly attached to the main body and boiler of the locomotive, while the front driver set is attached to the rear by a hinge, so that it may swing from side to side. The front end of the boiler rests upon a sliding bearing on the swinging front truck.
Mallets with a 2-8-8-2 wheel arrangement were a familiar sight on the Norfolk & Western Railroad. The Y3 models were built in 1919 by ALCO and Baldwin and were used to haul long coal trains over mountainous terrain.
Faced with motive power shortages during World War II, the Pennsylvania Railroad purchased six Alco-built Y3s from N&W in 1943. The Pennsy gave them the designation HH1 and used them in freight service until the late 1940s.
The O-gauge Mallet on my layout was made by MTH Electric Trains; I purchased it new in early 2002. The engine and tender are almost 26 inches long. It runs very smoothly (no jerky starts), makes a very powerful chugging sound and it's neat to watch those drivers and rods going back and forth. The MTH model is Protosound-equipped, producing realistic whistle and steam sounds.