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. They may also be referred to as a railcar or railmotor, depending on country. 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 none.
DMUs are usually classified by the method of transmitting motive power to their wheels.
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, 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.
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.
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:
In Queensland, heritage DMUs are used on the Savannahlander and Gulflander tourist trains.
Chinese manufactured(CNR TANGSHAN) DEMU was introduced in Bangladesh in from May 25, 2013. DEMU is the country's first-ever commuter train service started its trial journey on Chittagong-Fouzdarhat route.
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 (キハ43000系).
The service of several hundreds (in sum even thousands) of diesel railcars and DMUs started in 1950s following the improvement of fuel supply that was critical during World War II.
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.
Korail operates many DMUs. The DHC (Diesel Hydraulic Car), which made its debut for the 1988 Seoul Olympics, can reach speeds up to 170 km/h (106 mph) and serves Saemaul-ho trains.
Indian Railways operates DMUs in India.
DMUs were first introduced to Sri Lanka in 1940. Aim of this was connecting minor railway stations and stops on the main line where most of express trains were not stopping.
The DMUs are now usually used on the Taiwan Railway Administration Hualien–Taitung Line, North-Link Line. DMUs in Taiwan are classified as Class DR.
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 LNER 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 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, and the Class 121 "Bubble Car" is still in service as of 2015.
BR's owners, the British Government, 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 British Rail "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:
A type of Diesel Multiple Units in North America was the Budd Rail Diesel Car (RDC). The RDC was a single passenger car with two diesel engines and two sets of controls.
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. See Rail transport in Costa Rica.
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:
DMU manufacturers include: