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A diesel multiple unit or 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. Diesel-powered single-unit railcars are also generally classed as DMUs. Diesel-powered units may be further classified by their transmission type: diesel-electric (DEMU), diesel-mechanical (DMMU) or diesel-hydraulic (DHMU).
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.
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.
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.
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.
In modern DEMUs, such as the Bombardier Voyager family, each car is entirely self-contained and has its own engine, generator and electric motors. In older designs, such as the British Rail Class 207, 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.
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A train composed of DMU cars scales well, as it allows extra passenger capacity to be added at the same time as motive power. It also permits passenger capacity to be matched to demand, and for trains to be split and joined en route. It is not necessary to match the power available to the size and weight of the train, as each unit is capable of moving itself. As units are added, the power available to move the train increases by the necessary amount. DMUs may have better acceleration capabilities, with more power-driven axles, making them more suitable for routes with frequent closely spaced stops, as compared with conventional locomotive and unpowered carriage setups.
Distribution of the propulsion among the cars also results in a system that is less vulnerable to single-point-of-failure outages. Many classes of DMU are capable of operating with faulty units still in the consist. Because of the self-contained nature of diesel engines, there is no need to run overhead electric lines or electrified track, which can result in lower system construction costs.
Such advantages must be weighed against the underfloor noise and vibration that may be an issue with this type of train.
Generally diesel traction has several downsides compared to electric traction, namely higher fuel costs, more noise and exhaust as well as worse acceleration and top speed performance. The power to weight ratio also tends to be worse.
DMUs have further disadvantages compared to diesel locomotives in that they cannot be swapped out when passing onto an electrified line, necessitating either passengers to change trains or Diesel operation on electrified lines. Similarly the lost investment once electrification reduces the demand for diesel rolling stock is higher than with locomotive hauled trains where only the locomotive has to be replaced.
Diesel multiple units are in constant use in Croatia, operated by national operator Croatian Railways. On Croatian Railways, DMUs have important role since they cover local, regional and distant lines across the country. Two largest towns in Croatia, Zagreb and Split, are daily connected with DMU titling trains "RegioSwinger" (Croatian series 7123) that provide Inter City service between those two towns since 2004.
In the early 1990s, luxury DMU series 7021 (ordered for Yugoslav Railways in 1970s, now scrapped) provided some of higher ranked lines across the country. DMU series H? series 7121 (renumbered leftover from former Yugoslav Railways), 7122 (ordered from Sweden in late 1990's) and later Croatian-built series 7022 and 7023 (prototype-remained) are nowadays in high use covering country's local and regional services, mostly in country's interior on the tracks that are not fully electrified.
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.
The first significant use of DMUs in the United Kingdom was by the Great Western Railway, which introduced its small but successful series of diesel-mechanical GWR railcars in 1934. The London and North Eastern Railway and London, Midland and Scottish Railway 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 BR Regions laid down different specifications. The result was a multitude of different types, one of which was:
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 125 mph (201 km/h) trains run with a streamlined power car at each end and (typically) 7 to 9 intermediate trailer cars. Although originally classified as DEMUs, the trailer cars are very similar to loco-hauled stock, and the power cars were later reclassified as locomotives under Class 43. They remain in widespread 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 poor public image for the railway. A stopgap 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 '80s, 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 families of DMU:
Following the impact of the privatisation of British Rail in the late 1990s, several other diesel-hydraulic DMU families have been introduced:
In 2018 the first bi and tri-mode electro-diesel multiple units were introduced:
Canada generally follows similar buffer strength requirements to the USA, but new services are evaluated on a case-by-case basis. As a result, several types of lightweight DMUs have been used:
Costa Rica has purchased several Apolo 2400 series DMU railcars from the former narrow gauge operator in Spain, which are run in commuter service. 
A type of diesel multiple units in the U.S. was the Budd Rail Diesel Car (RDC). The RDC was a single passenger car with two diesel engines and two sets of controls.
In the United States only FRA-compliant DMU systems are permitted on freight rail corridors. This is due to the Federal Railway Administration setting higher coupling strength requirements than European regulators, effectively prohibiting the use of lighter weight European-style inter-city rail DMUs on U.S. main line railways without timesharing with freight operations or special waivers from the FRA. This has greatly restricted the development of DMUs within the U.S. as no other country requires the much heavier FRA compliant vehicles, and no export market for them exists.
Operations using FRA-compliant vehicles:
Operations using non FRA-compliant vehicles:
DMUs were first introduced to Australia in the late mid-20th century for use on quiet branch lines that could not justify a locomotive hauled service. Today, DMUs are widely used throughout Australia's southern states:
Chinese manufactured (CNR TANGSHAN) DEMU was introduced in Bangladesh from May 25, 2013. DEMU is the country's first-ever commuter train service which started its journey on the Chittagong-Fouzdarhat line.
The Indian Railways operates DMUs on its national network. The newly launched Diesel Electric Multiple Unit (DEMU) train will run from Delhi-Haryana.
State-owned company P.T. INKA builds several type of DMU, some of which operate in urban and suburban areas:
In Japan, where gasoline-driven railbuses (on small private lines) and railmotors (Kihani 5000 of the national railways) had been built since the 1920s, in 1937 the first two streamlined DMUs came in service, class Kiha 43000 (43000?).
The Southrail or the South Main Line of the Philippine National Railways which travels South of the Luzon island is one of the oldest rail lines in Asia and in the world. The Southrail of Philippine National Railways uses Hyundai Rotem DMUs together with second-hand DMUs from East Japan Railway Company or JR East. These are Kiha 52 and Kiha 59 which is also known as the "Kogane". Trains such as the Hyundai Rotem DMUs and Kiha 52 are often used for Metro Commuter Line services, while Kiha 59 is mostly used for Bicol interprovincial services and sometimes also for the Metro Commuter services.
DMU manufacturers include:
In the Voyager application, every car has a Cummins underfloor engine and alternator supplying power to a pair of body-mounted traction motors. Each drives one inner axle through a cardan shaft and axle-mounted final drive gearbox.