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A classic Belgian multiple unit of type 74

The term multiple unit or MU is used to describe a self-propelling train unit capable of coupling with other units of the same or similar type and still being controlled from one cab. The term is commonly used to denote passenger trainsets that consist of more than one carriage, but single self-propelling carriages, or railcars, can be referred to as multiple units if capable of operating with other units.

Multiple units are of three main types:

History and description

Multiple unit operation was made possible by the development of multiple-unit train control by the Americanmarker inventor Franklin J. Sprague. This allowed electrically-powered rapid transit trains to be operated from a single driving position.

The first successful test of an MU on a working rapid transit system was in Chicagomarker, on the South Side "L", now part of the CTA Green Line.

Most MUs are powered either by a diesel engine driving the wheels through a gearbox or hydraulic transmission (DMU), or by traction motors, receiving their power through a live rail or overhead wire (EMU). Diesel electric multiple units (DEMUs) have a diesel engine that drives a generator producing electricity to drive traction motors in a similar fashion to a diesel-electric locomotive.

A multiple unit trainset has the same power and traction components as a locomotive, but instead of the components concentrating in one carbody, they are spread out on each car that makes up the set. Therefore these cars can only propel themselves when they are part of the set; thus making them semi-permanently coupled. For example, a DMU might have one car carry the prime mover and traction motors, and another the engine for head end power generation; an EMU might have one car carry the pantograph and transformer, and another car carry the traction motors.

Virtually all rapid transit rolling stock, such as ones used in subway systems, are multiple unit trainsets, usually EMUs. Many high-speed rail rolling stocks are also multiple unit trainsets, such as the Japanesemarker Shinkansen and the Germanmarker ICE 3 high-speed trains.

Multiple units versus locomotive-hauled trains


Multiple units have several advantages over locomotive-hauled trains:

  • Energy efficiency — MUs are more energy efficient than locomotive-hauled trains. They are more nimble, especially on grades, as much more of the train's weight (sometimes all of it) is carried on power-driven wheels, rather than suffer the dead weight of unpowered hauled coaches. In addition, they have a lower weight-per-seat value than locomotive-hauled trains since they do not have a bulky locomotive that does not itself carry passengers but contributes to the total weight of the train. This is particularly important for train services that have frequent stops, since the energy consumed for accelerating the train increases significantly with an increase in weight.

  • Higher acceleration rate — Because of the energy efficiency, higher power - to - weight ratio and higher adhesive weight to total weight ratio values, MUs generally have higher acceleration ability than locomotive type trains and are favored in urban trains and subways for frequent start - stop routines.

  • No need to turn locomotive — Most MUs have cabs at both ends, resulting in quicker turnaround times, reduced crewing costs, and enhanced safety. The faster turnaround time and the reduced size (due to higher frequencies) as compared to large locomotive-hauled trains, has made the MU a major part of suburban commuter rail services in many countries. MUs are also used by most rapid transit systems. This is no longer a problem for locomotive hauled trains due to increasingly widespread usage of push-pull trains.

  • Makeup can be changed mid journey — MUs may usually be quickly made up or separated into sets of varying lengths. Several multiple units may run as a single train, then be broken at a junction point into smaller trains for different destinations.

  • Reliability — Due to having multiple engines, or motors, the failure of one engine does not prevent the train from continuing its journey. A locomotive drawn train typically only has one power unit whose failure will disable the train. Some locomotive hauled trains may contain more than one power unit and thus be able to continue at reduced speed after the failure of one.

  • Safety — Multiple units normally have completely independent braking systems on all cars meaning the failure of the brakes on one car does not prevent the brakes from operating on the other cars.

  • Axle load — Multiple units have lighter axle loads, allowing operation on lighter tracks, where locomotives are banned. Another side effect of this is reduced track wear, as traction forces can be provided through many axles, rather than just the four or six of a locomotive

  • Easy and quick driving — Multiple units generally have rigid couplers instead of the flexible ones on locomotive hauled trains. That means, brakes or throttle can be more quickly applied without excessive amount of jerk experienced in passenger coaches.

  • Allowance for accurate performance calculations for timetabling purposes - In a locomotive - hauled train, if number of cars is increased in order to meet the demand, acceleration and braking performance drops. This calls for the necessity that, the performance calculations are to be done taking the heaviest train composition into account. This may sometimes lead some trains in off- peak periods to be overpowered with respect to the required performance. But when two or more multiple units are coupled; train performance remains almost unchanged. However in locomotive hauled train compositions this problem can be solved by using more powerful locomotives when a train is longer.


Multiple Units do have some disadvantages as compared to locomotive hauled trains:

  • Maintenance - It may be easier to maintain one locomotive than many self-propelled cars.

  • Safety - In the past it was often safer to locate the train's power systems away from passengers. This was particularly the case for steam locomotives, but still has some relevance for other power sources. A head on collision or level-crossing accident involving a multiple-unit (with passengers potentially right at the front of the train) is likely to result in more casualties than one with a locomotive (where the heavy locomotive would act as a 'crumple zone').

  • Easy replacement of motive power - If a locomotive fails, it can be easily replaced with minimal shunting movements. There would be no need for passengers to evacuate the train. Failure of a multiple unit will often require a whole new train and time-consuming switching activities; also passengers would be asked to evacuate the failed train and board another one.

  • Efficiency - Idle trains do not waste expensive motive power resources. Separate locomotives mean that the costly motive power assets can be moved around as needed and also used for hauling freight trains. A multiple - unit arrangement would limit these costly motive power resources for use in passenger transportation.

  • Gangways - It is difficult to have gangways between coupled sets, and still retain an aerodynamic leading front end. Because of this fact, generally there is no passage between high-speed coupled sets.

  • Flexibility - Large locomotives can be substituted for small locomotives when more power is needed. Also, different types of passenger cars (such as reclining-seats, compartment cars, couchettes, sleepers, restaurant cars, buffet cars etc.) can be easily added to or removed from a locomotive hauled train. However, it is not so easy for a multiple unit since individual cars can be attached or detached only in a maintenance facility. This also allows a loco - hauled train to be flexible in terms of number of cars. Cars can be removed or added one by one, but in multiple units two or more units have to be coupled. This is not so flexible.

  • Crew resources - When two or more multiple units are coupled; since there would be no passage between them, crew(i.e ticket inspectors) should be present in all of them. This leads to higher crew costs and lower utilization of crew resources. In a locomotive - hauled train, one single crew can serve all the train regardless of the number of cars in the train unless limits of individual workload are not exceeded.

  • Buffet or restaurant cars - If presence of buffet or restaurant cars is a necessity; it may create a problem when two or more multiple units are coupled together. If there is no passage between the coupled sets, then buffets and restaurants should be present in all the sets individually. This reduces efficiency. In a locomotive - hauled train, one single buffet or restaurant can serve the entire train regardless of number of cars in the train, until its serving capacity is exceeded.

  • Noise and vibration - The passenger environment of a multiple unit is often noticeably noisier than that of a locomotive-hauled train, due to the presence of underfloor machinery. The same applies to vibration. This is a particular problem with DMU.

  • Obsolescence cycles - Separating the motive power from the payload-carrying cars means that either can be replaced when obsolete without affecting the other.


It is not necessary for every single car in an MU set to be motorized. Therefore MU cars can be motor units or trailer units. Instead of motors, trailing units can contain some supplemental equipment such as air compressors, batteries, etc.

In some MU trains, every car is equipped with a driving console, and other controls necessary to operate the train. Therefore every car can be used as a cab car whether it is motorised or not, if on the end of the train. This is the case with NJ Transit Arrows, Metro-North Railroad (New York) EMUs. However, other EMUs can be driven/controlled only from dedicated Cab cars. Among such EMUs are the former Russian ER2, ER9, German classes 423-426, etc.

Well-known examples of MUs are the Japanese Shinkansen and the last generation German ICE. Most trains in the Netherlandsmarker and Japanmarker are MUs, making them suitable for use in areas of high population density. A new high-speed MU, the AGV, was unveiled by France's Alstom on February 5, 2008. It has a claimed service speed of 360 km/h.

By country


In the Republic of Irelandmarker the railway operator Iarnród Éireann has purchased a number of new DMUs since 1993 to replace older locomotives and carriages.


In Japanmarker, most passenger train vehicles including the high-speed Shinkansen are Multiple unit type except for small numbers of overnight sleeper trains — very few passenger trains are now locomotive type. Well over 90 per cent of passenger trains are currently MU type.

Japan is a country of high population density and large number of railway passengers in a relatively small urban area, and frequent operation of short distance trains has been required. Therefore, the high acceleration ability and quick turnaround times of MU have advantages, encouraging their development in this country.

Most long distance trains in Japan had been operated by locomotives until the 1950s, but by utilizing and enhancing the technology of short distance urban MU trains, long distance express MU type vehicles were developed and widely introduced starting in the mid 1950s. This work resulted in the development of the MU type high-speed train, the Shinkansen, in 1964. Later on, locomotive type trains have been regarded as slow and inefficient, and their use has significantly decreased in Japan.


Elektrichka ( , ) is an informal word for elektropoezd ( ), a Sovietmarker or post-Soviet regional (mostly suburban) electrical multiple unit passenger train. Elektrichkas are widespread in Russiamarker, Ukrainemarker and some other countries of the former Soviet Union. The first elektrichka ride occurred in August 1929 between Moscowmarker and Mytishchimarker.

United Kingdom

In Northern Irelandmarker the majority of passenger services have been operated by diesel multiple units since the mid-1950s under the tenure of both the Ulster Transport Authority (1948-1966) and Northern Ireland Railways (since 1967). In the UK the use of modern diesel multiple units was pioneered in Northern Ireland, although a number of other railway companies also experimented with early DMUs (including the Great Western and the London Midland Scottish). Notable examples include the Sprinter and Voyager families, and the brand new Olympic Javelin train service.

United States and Canada

Most long-distance trains in North America are locomotive-hauled. However, commuters, rapid transit, and light rail operations make extensive use of MUs. Most electrically powered trains are MUs. The Southeastern Pennsylvania Transportation Authority (SEPTA) Regional Rail Division uses EMUs almost exclusively — the exception being some of its peak express service. New Jersey Transit service on the Northeast Corridor Line is split between electric locomotives and EMUs.

M2, M4, M6 and future M8 EMUs which operate on the New Haven Line of Metro-North Railroad, are “dual mode” meaning they can draw power from either the third rail or from overhead lines. This allows operation under the wires between Pelham, NYmarker and New Haven, CTmarker, a section of track owned by Metro North but shared with Amtrak's Northeast Corridor service, and on third rail between Pelham and Grand Central Terminalmarker. EMUs are used on AMT's Montreal/Deux-Montagnes line.

DMUs are less common, partly because new light rail operations are almost entirely electric, with many commuter routes already electrified, and also because of the difficulties posed by Federal Railway Administration rules limiting their use on shared passenger/freight corridors. When the Budd RDC was developed following World War II, it was adopted for many secondary passenger routes in the United States (especially on the Boston and Maine Railroad) and Canada. These operations generally survived longer in Canada, but several were abandoned in the VIA Rail cutbacks of the early 1990s. One that survives is The Mahalatmarker on Vancouver Island.

DMUs are used on the RiverLINEmarker in New Jersey. Currently Colorado Railcar is demonstrating an FRA Crash Compliant DMU in the United States. NJ Transit has experimented with this DMU on the Princeton Branch line. In August 2006 it was announced that Amtrak wants the State of Vermont to experiment with DMUs on the state-subsidized Vermonter line from New Havenmarker north to St. Albans to replace the less efficient diesel locomotive trainsets currently used.

Freight multiple units

A new concept is to use the multiple unit idea for freight traffic, such as carrying containers or for trains used for maintenance. The Japanese M250 series train has four front and end carriages that are EMUs, and has been operating since March 2004. The German CargoSprinter have been used in three countries since 2003.


The United Kingdom had various examples of Autotrain on branch lines, whereby a steam locomotive could be controlled by driving trailers at the opposite end of the train. This avoided the need to run around the locomotive at the terminals. These autotrains were limited to about two carriages.


See also



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