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The term Multiple Unit or MU is used to describe a self propelling train unit capable of coupling with other like units and be controlled from one cab. The term is most often used on passenger trainsets that consists 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. More commonly used in North America, the term can also refer to multiple locomotive operation. The driver's cab on a MU is usually truncated to a short room at each end of the train.

Multiple units are of two main types:

• Electric Multiple Unit (EMU)
• Diesel Multiple Unit (DMU)

Contents 1 History and description 2 Trainset 3 Multiple Units vs. Locomotives 3.1 Advantages 3.2 Disadvantages 4 Features 5 Ireland 6 North America 7 Russia 8 See also

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

Before his successful invention, differences in the speed and response of motors on different cars of the train caused binding on the couplings between the train cars, wheel slippage and excess wear on motors due to operating gear running at speeds faster or slower than the overall speed of the train, or even derailment, as well as an uncomfortable ride.

Most MUs are powered either by a diesel engine driving the wheels through a gearbox or hydraulic transmission (DMU), or by electric 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 electric motors in a similar fashion to a Diesel-electric locomotive.

Virtually all rapid transit rolling stock are multiple unit trainsets, usually EMUs. Famous multiple unit trainsets include the Japanese Shinkansen and the German ICE 3 high-speed trains.

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 complete trainset, 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.

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;

• No need to turn locomotive - 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 underground railways.

• Consist 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 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, such as the Whitby line. Another side effect of this is reduced track wear, as traction forces can be provided through many axles, rather than just four or six.

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.

• Easy replacement of motive power - Should a locomotive fail, it is easily replaced. Failure of a multiple unit train-set will often require a whole new train.

• 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.

• Flexibility - Large locomotives can be substituted for small locomotives where the gradients of the route become steeper and more power is needed.

• Noise - The passenger environment of a multiple unit is often noticeably noisier than in a locomotive-hauled train, due to the presence of underfloor machinery. This is a particular problem with DMUs.

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

• It is difficult to have gangways between coupled sets, and retain an aerodynamic leading front end.

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

In some EMU 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, etc. However, other EMUs can be driven/controlled only from dedicated Cab cars. Among such EMUs are Ex-USSR-made 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 Netherlands and Japan are MUs, making them suitable for use in areas of high population density. Even some freight trains are MUs: the M250, whose four front and end carriages are EMUs, has been operating in Japan since March 2004.

EMUs are also extensively used in all local train networks in India that provide commuter transport in all the major cities.

See Multiple Units of Ireland

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

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, NY and New Haven, CT, 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 Terminal. 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. DMUs are used on the RiverLINE in New Jersey, and there are efforts to develop effective passenger DMUs for inter-city trains. NJ Transit has also experimented with DMUs on the Princeton Branch (a.k.a. Princeton "Dinky") 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 Haven north to St. Albans to replace the less efficient diesel locomotive trainsets currently used.

See Elektrichka.

• Air brake (rail)
• Railway brakes
• Diesel locomotive
• Push-pull (mode of operation for locomotive-hauled trains)
• Rail terminology
• Multiple-unit train control
• Pacer (train) units.
• Railcar
• Hybrid Locomotive