A diesel multiple unit (DMU) is a multiple unit train powered by on-board diesel engines. A DMU requires no separate locomotive, as the engines are incorporated into one or more of the carriages. When additional carriages are coupled on, their controls are connected through and a single driver can control every engine in the train. This also allows the driver to drive from a cab at either end, simplifying reversing. Diesel-powered single-unit railcars are generally regarded as DMUs for most operations, at least with smaller trains.

Advantages The first diesel railcars appeared in the 1930s and DMUs became popular immediately post-war. They offered several cost-saving advantages, at a time when railway operations were short of funds: Greater convenience of operation. They could go from parked and stabled to running a train in little time, unlike a steam locomotive that needed to be prepared hours in advance, then disposed afterwards. They could also run trains intermittently through the working day without needing to tend their fire or boiler even when idle. Reduced crew. They could be driven by a single driver, without a fireman. Simpler station layouts. They did not need a locomotive to 'run round' the train at a terminus station, merely for the driver to change ends. This made them quicker to operate in a busy station. It also permitted small stations to be simplified from a loop to a simple dead-end, also saving on signalling costs and the need to employ a signaller. This simplicity could be enough to keep small branch lines in viable operation, rather than being closed. Greater flexibility. As each carriage or pair of carriages was powered, more carriages could be added to a longer train and there was no need to swap in a more powerful locomotive.

Design The diesel engine may be located above the frame in an engine bay or under the floor. Driving controls can be at both ends, on one end or in a separate car.

Types by transmission DMUs may also be classified by the method of transmitting motive power to their wheels: diesel–mechanical DMMU, diesel–hydraulic DHMU, or diesel–electric DEMU.

Diesel–mechanical In a diesel–mechanical multiple unit (DMMU), the rotating energy of the engine is transmitted via a gearbox and driveshaft directly to the wheels of the train, like a car. The transmissions can be shifted manually by the driver, as in the great majority of first-generation British Rail DMUs but, in most applications, gears are changed automatically.

Diesel–hydraulic

In a diesel–hydraulic multiple unit (DHMU), a hydraulic torque converter, a type of fluid coupling, acts as the transmission medium for the motive power of the diesel engine to turn the wheels. Some units feature a hybrid mix of hydraulic and mechanical transmissions, usually reverting to the latter at higher operating speeds as this decreases engine RPM and noise.

Diesel-hydraulic locomotive transmissions can be hard to distinguish from diesel-mechanical, where both have multiple mechanical ratios and a single torque converter. This is especially true for lower powers (<500 hp) such as DMUs. Some railways, such as British Rail in the 1960s, retreated from using hydraulic transmissions from main line locomotives, yet had no difficulty with large DMU fleets based on torque converters.

Diesel–electric

In a diesel–electric multiple unit (DEMU), a diesel engine drives an electrical generator or an alternator, which produces electrical energy. The generated current is then fed to electric traction motors on the wheels or bogies in the same way as a conventional diesel–electric locomotive.

On some DEMUs, such as the Bombardier Voyager, each car is entirely self-contained and has its own engine, generator and electric motors. In other designs, such as the Class 207 or the Stadler GTW and Stadler FLIRT DMU,Stadler Flirt DMU/DEMU example: Elron Elektriraudtee Classes 2200, 2300 and 2400, TEXRail Stadler FLIRT some cars within the consist may be entirely unpowered or only feature electric motors, obtaining electric current from other cars in the consist which have a generator and engine.

Around the world Europe Belgium NMBS/SNCB uses its NMBS/SNCB Class 41 DMUs on the few remaining unelectrified lines. As electrification progresses, the DMUs have become less utilised.

Croatia

Diesel multiple units cover large number of passenger lines in Croatia, which are operated by the national passenger service operator HŽ Putnički prijevoz. On Croatian Railways, DMUs have important role since they cover local, regional and distant lines all across the country. The country's two largest towns, Zagreb and Split, are connected with an inter-city service that is provided by DMU tilting trains "RegioSwinger" (Croatian series 7123) since 2004. Those trains may also cover other lines in the country, depending on need and availability.

Luxury DMU series 7021, built in France, started to operate for Yugoslav Railways in 1972 and, after 1991, still remained in service of Croatian Railways until 2005. Units 7121 and 7122 (which came as a replacement for 7221 units), together with the newest series 7022 and 7023 built in 2010s Croatia, cover many of the country's local and regional services on unelectrified or partly electrified lines.

Czech Republic

Diesel multiple units also cover large number of passenger lines in the Czech Republic which are operated by the national operator České dráhy. They have an important role since they cover local, regional and distant lines all across the country. Those trains may also cover other lines in the country depending on need and availability too.

Also, the DMUs were manufactured for foreign carriers. The tables of cars and units are divided into vehicles operated until 1987, when the ČSD used the series designations proposed by Vojtěch Kryšpín, and vehicles created after this date, which no longer have Kryšpín's designations (with some exceptions). In addition, these new cars are the new vehicles are already different in both countries.

Estonia Elron has since 2015 a Stadler FLIRT fleet, with 20 trains DEMU version.

Germany Germany has employed DMUs for both commuter and express services for many decades. The SVT 877 Fliegender Hamburger DMU introduced in 1933 made the run from Berlin to Hamburg in a record-setting 138 minutes, and its derivative SVT 137 broke the land speed record in 1936. After World War II, the VT 11.5 DMU was the flagship of the glamorous Trans Europ Express.

Since 1968, Deutsche Bahn (DB) has designated DMUs with class numbers beginning in 6. While DB and regional transport authorities generally prefer electric power for commuter rail, many local and rural lines remain unelectrified and DMUs are needed for those areas. DMUs in service as of 2021 include the Adtranz Class 612 tilting train ("Regio Swinger"), the Alstom Coradia LINT (Classes 620–623, 640 and 648), the Siemens Desiro (Class 642) and the Bombardier Talent (Class 643/644). From 2001 to 2016 there was even a DMU version of DB's high-speed Intercity Express, the Class 605 ICE TD.

Greece Diakopto–Kalavryta railway

Ireland

In the Republic of Ireland, the Córas Iompair Éireann (CIÉ), which controlled the republic's railways between 1945 and 1986, introduced DMUs in the mid-1950s and they were the first diesel trains on many main lines.

Italy

DMUs were introduced in Italy around the 1930s, when Fiat produced the first unit of the ATR 100 series; this was a fixed-composition train that was innovative for the time, equipped with two diesel-powered extreme traction units with a trailered intermediate unit. They are still used today.

Romania

DMUs are used mostly on shorter or less frequently travelled routes in remote areas. The national railway company CFR still uses, along with other DMU models, Class 77 and 78 DMUs, locally built by Malaxa between the 1930s and 50s and refurbished in the 70s. The main DMU in use is the Class 96 Siemens Desiro aka Săgeata Albastră (The Blue Arrow). Private operators also largely use DMU units, mainly purchased from various French and German operators.

Slovakia

Several types of DMUs operate in Slovakia; the most common is the Class 812 ZSSK, based on the ČD Class 810. These are used almost exclusively for hauling passenger trains on non-electrified regional lines and excel at low travel speeds. In the past, there were a number of express trains driven by motor coaches, which often overcame heavier trains driven by steam locomotives at cruising speed. A typical example is the Slovenská strela motor express train, led on the Bratislava-Prague route by a motor car of the same name, or the Tatran express from Bratislava to Košice. Representatives of high-speed motor wagons were motor wagons of the M262 or M286 series, which lost their application in high-speed wagons; this was due to the gradual electrification of main lines and are used for passenger trains.

United Kingdom

The first significant use of DMUs in Great Britain was by the Great Western Railway, which introduced its small but successful series of diesel–mechanical GWR railcars in 1934. The London & North Eastern Railway (LNER) and London, Midland & Scottish Railway (LMS) also experimented with DMUs in the 1930s; the LMS both on its own system and on that of its Northern Irish subsidiary, but development was curtailed by World War II.

After nationalisation, British Railways (BR) revived the concept in the early 1950s. At that time, there was an urgent need to move away from expensive steam traction which led to many experimental designs using diesel propulsion and multiple units. The early DMUs proved successful and, under BR's 1955 Modernisation Plan, the building of a large fleet was authorised. These BR first generation DMUs were built between 1956 and 1963.

BR required that contracts for the design and manufacture of new locomotives and rolling stock be split between numerous private firms, as well as BR's own workshops, while different regions laid down different specifications. The result was a multitude of different types, one of which was: 'Intercity' units, which were more substantially constructed and shared many features with contemporary hauled coaching stock. They were built for express services on important secondary routes on the Scottish, North Eastern and Western regions.

In 1960, British Railways introduced its Blue Pullman high-speed DEMUs. These were few in number and relatively short-lived, but they paved the way for the very successful InterCity 125 or High Speed Train (HST) units, which were built between 1975 and 1982 to take over most principal express services on non-electrified routes.

These trains run with a streamlined power car at each end and typically seven to nine intermediate trailer cars. Although originally classified as DEMUs, the trailer cars are very similar to locomotive-hauled stock and the power cars were later reclassified as locomotives under Class 43. HSTs started being replaced in 2017 but, as of October 2022, some are still in use.

By the early 1980s, many of the surviving first generation units were reaching the end of their design life, leading to spiralling maintenance costs, poor reliability and a negative public image for the railway. A stop gap solution was to convert some services back to locomotive haulage, as spare locomotives and hauled coaching stock were available, but this also increased operating costs. Commencing in the mid 1980s, British Rail embarked upon its so called "Sprinterisation" programme, to replace most of the first generation DMUs and many locomotive-hauled trains with three new types of DMU: Class 140–144 Pacer railbuses, ultra-low-cost diesel–mechanical units (later converted to hydraulic) utilising four-wheeled chassis and lightweight bus bodywork, designed for provincial branch line and stopping services.