DMU

A Diesel Multiple Unit, abbreviated DMU, is a self-propelled passenger train where each car (or every second car, depending on the design) carries its own diesel engine and transmission, requiring no separate locomotive. Power is mostly applied via diesel-mechanical or diesel-hydraulic drives on commuter-grade DMUs and diesel-electric drives on heavier intercity sets, where the diesel turns a generator and the generated current drives traction motors on the bogies — essentially an EMU with its own onboard power station.

DMUs dominate non-electrified regional and rural networks where the cost of installing catenary cannot be justified by traffic levels. British First Generation DMUs from the 1950s — the Class 101s, 108s, 121s — replaced steam-hauled local trains across thousands of route miles and ran into the 1990s. Their modern descendants — the Class 165s, 170s, 175s, 195s, the German Alstom Lints, the French AGCs, the American Stadler FLIRT DMUs — still cover branch lines and secondary main lines worldwide.

The recent appearance of battery-electric multiple units (BEMUs) and hydrogen multiple units (HMUs) is gradually displacing DMUs on routes where electrification remains uneconomic but emissions matter. The transition will be slow — DMUs typically have 30-year service lives and the existing fleet is not yet old — but DMUs are the most likely intermediate technology to be retired in the 2030s as decarbonisation rolls out.

For railfans, DMUs are the secret backbone of many photogenic rural and regional networks: lighter than locomotive-hauled trains, faster-accelerating, and frequently coupled into multi-unit sets on busier branches where length matters. A two-car DMU on a single track threading hills is one of the most recognisable signatures of European and British secondary railway operation.