A multiplex locomotive is a steam locomotive that divides the driving force on its wheels by using multiple pairs of cylinders to drive multiple driving wheel set groups. Such a locomotive will necessarily articulated if it has more than two sets of driving wheels. There were locomotive projects with three, four, five or six sets of drive wheels. However, these locomotives were never built, except for four triplex locomotives in the United States and one quadruplex locomotive in Belgium.
The names of the subgroups of multiplex locomotives is derived from the number of drive wheel sets and is composed of the numeral prefix and the word locomotive. However, locomotive is often omitted if it can be inferred from the context.