A diesel multiple unit or DMU is a multiple unit train consisting of multiple carriages powered by one or more on-board diesel engines. They may also be referred to as a railcar or railmotor, depending on country.
DMUs are usually classified by the method of transmitting motive power to their wheels.
In most modern DEMUs, each car is entirely self-contained and has its own engine, generator and electric motors. In older designs some cars within the consist may be entirely unpowered or only feature electric motors, obtaining electrical current from other cars in the consist which have a generator and engine.
Direct-drive diesel locomotives often require an impractical number of gears to keep the engine within its powerband; coupling the diesel to a generator eliminates this problem. Power still needs to be transmitted to the generator or alternator via a simple gearbox but this is advantageous because;
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.
DMUs were first introduced to Australia in the early 20th century for use on quiet branchlines that could not justify a locomotive hauled service. Today a range of modern DMUs are extensively used across Australia for both commuter and intercity routes:
The Flying Hamburger of Germany, introduced 1933, established the then-time fastest regular railway connection of the world. Top speed was 160 km/h, the average speed being 124 km/h on the tracks between Berlin and Hamburg.
The Trans Europ Express travelled international traffic between countries like Germany, France, Italy, Netherlands in the 1950s and 1960s. They were diesel multiple units since the electrical systems varied a lot.
Also from Germany is the CargoSprinter concept. With two motorized units and three flatbed wagons between them, this DMU container train tried to compete with container road trucks by profiting from existing railway access to factories and businesses, but eliminating the need for inflexible locomotive-pulled cargo trains. The payload was 160 tons. Suffering from technical problems and failing political support for short-haul cargo railway connections, the prototypes were sold to Austria.
Currently DMUs are used in large numbers for local traffic (e.g. DB Class 642) and fast, tilting regional traffic (e.g. DB Class 612). The introduction of DB Class 605 as a diesel variant of the tilting EMU DB Class 411/415 in 2001 was not successful.
In the Republic of Ireland the Córas Iompair Éireann (CIE), which controlled the republic's railways between 1945-86, introduced DMUs in the mid-1950s and they were the first diesel trains on many main lines. The Great Northern Railway, merged into CIE at this time, also brought some DMUs of its own of a similar style. They were not well suited to the long-distance tasks and were replaced in a few years by traditional trains with new diesel locomotives, being then mostly restricted to Dublin commuter lines. The power systems were worn out by the 1970s so they were converted to normal carriages pulled by diesel locomotives on these suburban routes, and DMUs disappeared from CIE. But since 1987, Iarnród Éireann (IE) have been increasing the use of this type of train, in order to replace older locomotives and carriages, using new types of train manufactured in a number of overseas countries.
In Northern Ireland the Northern Ireland Railways used DMUs extensively from the 1950s throughout its system, notably the NIR 80 Class introduced in the early 1970s and currently being replaced (as of 2007).
The development of DMUs in Japan started in 1950s following the improvement of fuel supply that was critical during World War II. In 1953, the Japanese National Railways put the hydraulic torque converter into practical use. This invention facilitated the development of DMUs, which spread all over the nation as not only on local services but also on long distance express services. In 1960, the 9-car KiHa 80 series DMUs debuted on Hatsukari limited express service connecting Tokyo and Aomori (about 750 km).
Presently, all the passenger railway companies of Japan Railways Group (JR Group) operate DMUs on semi-trunk and local lines while almost all of trunk lines have been electrified. On the JR lines not yet electrified, locomotive trains were abandoned but only small number of sleeper trains. There are also a number of independent railway lines that operate DMUs.
One of the first 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. Any number of cars could be connected together and all the propulsion systems controlled from a single operator's station or cab. Introduced in the early 1940s, the cars were used on rural railway lines that did not warrant full passenger trains, or short commuter services.
Several rail operators in the United States use DMUs suitable for mainline use:
Canada generally follows similar buff 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:
Korail operates many DMUs. The DHC (Diesel Hydraulic Car), which made its debut for the 1988 Seoul Olympics, can reach speeds up to 150 km/h and serves Saemaul-ho trains. The NDC (New Diesel Car) serves Mugunghwa-ho trains and the 통근형 디젤 동차 (Commute Diesel Car) serves Tonggeun trains. The NDC and CDC can reach speeds up to 100 km/h and are not used for mountainous lines such as the Taebaek Line.
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, Midland and Scottish Railway also experimented with DMUs in the 1930s, 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 some are still in service as of 2008. Most were diesel-mechanical, but a few designs had hydraulic transmissions (these were generally less successful and were withdrawn earlier than the main diesel-mechanical types).
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, some of which were built only in small numbers, but in general the First Generation units fell into five distinct groups:
There were also a small number of four-wheeled railbuses built for the most lightly-used branch lines, but these failed to prevent the closure of such lines, and all of the railbuses had gone by the end of the 1960s.
Diesel Electric Multiple Units (DEMUs) were also developed during the 1950s and 1960s, for use on the Southern Region. The Southern was largely electrified, but required diesel units for the remaining lines, in some cases as stop-gaps pending planned electrification. Diesel-mechanical and diesel-hydraulic units were judged to have inadequate acceleration, which would have caused delays to other traffic when operated over electrified lines. Examples of Southern DEMUs included Classes 205 and 207, which were nick-named "Thumpers" because of their characteristic sound.
At the end of the 1950s, 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 1984 to take over most principal express services on non-electrified routes. These 125mph trains run with a streamlined power car at each end and (typically) 7 or 8 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 becoming life-expired, which lead 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 privatisation of British Rail in the late 1990s, several other diesel-hydraulic DMU families have been introduced:
As modern diesel-hydraulic units have sufficient performance to match the acceleration of Electric Multiple Units, they have replaced DEMUs on the former Southern Region local/commuter services. However the vast majority of British non-electric InterCity services are currently operated by Diesel Electric Multiple Units, the HSTs having been joined since privatisation by high speed Bombardier Class 220 Voyager, Class 221 Super-Voyager and Class 222 Meridian/Pioneer express units.
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