A DEMU train is a multiple-unit train powered by onboard diesel engines. Unlike traditional trains, a DEMU does not require a separate locomotive, as the engines are incorporated into one or more of the carriages. DEMUs are widely used in India, where they have replaced many older trains with a WDM-2 or WDM-3A locomotive in the middle. DEMUs are also used in other countries, such as Bangladesh and Cambodia.

The primary difference between a DEMU and an EMU (Electric Multiple Unit) is the source of power. In a DEMU, the primary source of power is diesel, while in an EMU, the primary source of power is electricity collected from the overhead catenary.

DEMU trains are designed to facilitate short-distance travel on non-electrified routes. They are self-propelled and have a higher acceleration and retardation compared to traditional locomotive-hauled trains, which enables them to serve smaller stations with quick start and stop functionality.