 A multiple unit trained or simply multiple unit mute is a self-propelled train composed of one or more carriages joined together, which when coupled to another multiple unit can be controlled by a single driver. Although most multiple units consist of several carriages, single self-propelled carriages also called rail cars, rail motor coaches or rail buses are multiple units if they are capable of operating with others. Multiple units are classified by their power source and are of two main types, electric multiple unit mute or diesel multiple unit DMU. Diesel powered units may be further classified by their transmission type, diesel electric DMU diesel mechanical DMMU or diesel hydraulic DHMU. The term multiple unit does not denote locomotives using multiple unit train control.