A Mallet locomotive
is a type of articulated locomotive. A Mallet locomotive
has a long boiler and double propulsion units - the rear unit is directly attached to the main body of the locomotive, while the front unit is mounted on a bogie coupled to the front of the boiler, the end of which overhangs on tight curves.
In Whyte notation Mallet locomotives are expressed using a 4-part notation.
The most famous Mallet locomotive is the Union Pacific Big Boy, which had a 4-8-8-4 wheel arrangement.