EMU vs. DMU: Understanding Multiple Units

In 1954, British Railways introduced the first diesel multiple units on rural branch lines that could not justify the cost of electrification. The idea was straightforward: instead of sending a locomotive and a rake of coaches to carry 40 passengers to a market town, send a self-contained diesel railcar that needs no engine change at the terminus and can turn around in minutes. The operating cost savings were immediate and substantial.

Seventy years later, that same logic still governs rolling stock procurement on railways worldwide. The multiple unit — whether electric, diesel, battery, or hydrogen — has displaced the locomotive-hauled train on virtually every passenger service type except long-distance intercity and overnight travel. Understanding the differences between EMU, DMU, and the emerging alternative traction variants is essential to understanding how modern railway networks are specified, procured, and operated.

What Is a Multiple Unit?

A multiple unit is a train consisting of self-propelled vehicles whose traction equipment — motors, power electronics, and in the case of DMUs, engines — is distributed across the trainset rather than concentrated in a separate locomotive. Each powered vehicle carries its own traction package, and the vehicles are semi-permanently coupled into fixed formations (units) that operate as a single train.

The “multiple” in the name refers to the ability to couple two or more units together and operate them as a single train under the control of one driver — a capability that allows operators to flexibly increase capacity on busy services by joining units, and reduce it by splitting them.

EMU, DMU, BEMU, HMU: The Full Taxonomy

EMU vs DMU: A Detailed Comparison

Multiple Unit vs Locomotive-Hauled: Why MUs Won

The shift from locomotive-hauled trains to multiple units on passenger services accelerated from the 1980s onwards and is now essentially complete in most markets. The operational advantages are decisive:

Terminal turnaround: A locomotive-hauled train arriving at a terminal station requires the locomotive to be uncoupled and run around to the other end of the train — a process taking 10–20 minutes minimum. A multiple unit simply has the driver walk to the other cab. On high-frequency urban services with 5-minute turnarounds, this difference alone justifies the switch.

Distributed power: A locomotive concentrates all traction on two or three powered axles at one end of the train. A multiple unit spreads traction across many powered bogies throughout the train, giving better adhesion in poor rail conditions, higher acceleration, and redundancy against traction equipment failure.

Capacity density: A locomotive occupies 20–25 metres of train length and carries zero passengers. In a multiple unit, every metre of the train contributes to passenger capacity.

Locomotive haulage retains advantages in specific niches: very long trains (freight, overnight passenger), operations requiring frequent formation changes (adding/removing vehicles to match demand), and routes where the same locomotive serves multiple train types.

How a Multiple Unit Formation Works

A multiple unit formation consists of different vehicle types combined in a fixed or semi-fixed arrangement:

A typical 4-car suburban EMU might be arranged as DM-T-T-DM (powered ends, unpowered middle) or DM-M-M-DM (all powered). High-speed trains often have a higher proportion of motor vehicles for maximum power output. The ratio of motor to trailer vehicles directly affects the train’s power-to-weight ratio, acceleration, and electricity consumption.

BEMU: The Technology Replacing DMUs

The Battery Electric Multiple Unit is the most significant new rolling stock category of the 2020s. It operates as an EMU on electrified sections — collecting current from the overhead wire or third rail — and automatically switches to battery power when entering non-electrified territory, continuing operation without any change perceptible to passengers.

The battery is recharged during electrified running and through regenerative braking. On a typical regional route with a mix of electrified main line and non-electrified branch, a BEMU can complete the entire route cycle with the batteries charged and ready for the next service.

Key deployed BEMU examples as of 2026:

• Stadler FLIRT Akku — operating in Germany; up to 80 km battery range
• Alstom Coradia Continental battery — deployed on several German regional networks
• Hitachi AT-300 bi-mode — used as Class 802 in the UK; diesel + electric with battery buffer
• CAF Civity battery — deployed in Spain and increasingly in UK
• Siemens Mireo Plus B — battery-only variant for fully non-electrified operation

Real-World Examples by Type

The Declining DMU: What Replaces It?

New diesel-only multiple unit orders have declined sharply since 2020. The reasons are straightforward: tightening EU emissions regulations (Stage V engine standards), operator decarbonisation commitments, and the improving economics of battery technology have made new DMU procurement increasingly difficult to justify.

The replacement pathway depends on route characteristics:

• Short non-electrified branches off electrified main lines → BEMU (charge on the main line, run on battery on the branch)
• Longer non-electrified routes where battery range is insufficient → Hydrogen MU or bi-mode diesel-electric
• Routes where full electrification is planned within 10 years → BEMU as bridge technology, then pure EMU
• Very lightly used rural lines → Hydrogen or battery single-car units

Editor’s Analysis

Frequently Asked Questions

Q: What does “multiple unit” mean — can you run more than one together?

Yes — “multiple unit” refers to both the self-propelled nature of the train and its ability to couple with other units and operate as a single longer train under one driver. Two 4-car units coupled together form an 8-car train, with the lead driver controlling all traction and braking via a system called Multiple Unit Working (MUW). This allows operators to run longer trains at peak times and shorter trains off-peak, using the same vehicles, without changing formation manually.

Q: Why do EMUs accelerate faster than locomotive-hauled trains?

EMUs accelerate faster because their traction motors are distributed across many powered bogies throughout the train, giving a high power-to-weight ratio. A locomotive concentrates all power in one vehicle that must accelerate both itself and the unpowered coaches behind it. An EMU applies traction force across every powered axle simultaneously, and because every vehicle contributes to traction, there is more adhesion available before wheel slip occurs. On a stop-start suburban service, this difference in acceleration can save 30–60 seconds per station stop.

Q: What is the difference between a BEMU and a bi-mode train?

A BEMU (Battery Electric Multiple Unit) uses only electric traction motors throughout — it charges its batteries from an overhead wire or third rail and discharges them on non-electrified sections. It has no combustion engine. A bi-mode train combines two different power sources — typically electric from overhead wire and diesel from on-board engines — and switches between them as needed. Bi-modes are heavier and have higher maintenance costs than BEMUs because they carry both electric and diesel traction systems, but they can operate on any route regardless of battery range.

Q: How is a hydrogen train different from a battery train?

Both use electric traction motors, but the energy is stored differently. A battery train stores electricity directly in lithium-ion or similar battery cells. A hydrogen train generates electricity on board by passing hydrogen through a fuel cell, which produces electricity and water as a by-product. The hydrogen is stored in high-pressure tanks on the roof. Hydrogen offers greater energy density by weight than current batteries — allowing longer range between refuelling — but requires dedicated hydrogen refuelling infrastructure at depots. Battery trains are simpler and benefit from a larger global supply chain.

Q: Are high-speed trains like the TGV also multiple units?

Yes. The TGV, Eurostar, ICE, Shinkansen, and most high-speed trains are technically electric multiple units, though their internal arrangement varies. The original TGV used a semi-distributed approach — two power cars at each end (similar to locomotives) with all intermediate vehicles being unpowered trailers connected by Jacobs bogies. Modern high-speed trains like the Siemens Velaro use fully distributed traction with motors throughout the train. In all cases, the trains are self-propelled and do not require a separate locomotive — the defining characteristic of a multiple unit.