Multiple unit

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Image:AM73.jpg The term Multiple Unit (MU) is usually used to describe a passenger train whose carriages have the ability to propel on their own, can be coupled with other similar units to operate together in multiple, and the minimum operating train unit consists of more than one carriages. Depending on the energy source, the two major categories of multiple-unit train are Diesel Multiple Unit (DMU) and Electric Multiple Unit (EMU). If the minimum operating unit consists of only one carriage, it is usually called a Rail Car.

A multiple unit train consist has the same power and traction components as a locomotive, but instead of the components concentrating on one carbody, they are spread out on each car that make up the consist. Since the power and traction components are spread out on each car, these cars can only propel themselves when they are within one complete consist, thus make them semi-permenantly connected. For example, a DMU might have one car carry the prime mover and traction motors, and another carry another engine for head end power generation; an EMU might have one car carry the pantograph and transformer, and another car carry the traction motors.

More commonly used in the United States, the term Multiple Unit can also refer to the capability of many locomotives in one train to be operated at one location, either at a leading locomotive or at a cab car. This enables locomotives to be place anywhere within the train, operating in a push-pull method. See multiple-unit train control for more information.

1 History and description
2 Multiple Units vs. Locomotives
- 2.1 Advantages
- 2.2 Disadvantages
3 EMU features
4 Commuter Rail
5 North America
6 See also

[edit] History and description

Multiple units were made possible by the development of multiple-unit train control by the American inventor (Franklin J. Sprague), to allow electrically-powered rapid transit trains to be operated from a single position without the need for a separate locomotive, as was required when such trains were hauled by steam engines.

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 and operating gear running at speeds faster or slower than the overall speed of the train, or even derailment, as well as an uncomfortable ride.

The motors driving the train on an MU unit are typically mounted underneath the floor of the carriages, on the bogies (U.S. "trucks"), the assembly beneath the train that holds the axles and wheels.

The driver's cab on an MU is usually truncated to a short room at both ends of the train.

Most MUs are powered either by a diesel engine driving the wheels through a gearbox or hydraulic transmission (a diesel multiple unit, or DMU), or by electric motors, receiving their power through a live rail or overhead wire (an electric multiple unit or EMU). Diesel electric multiple units (DEMUs) also exist: these have a diesel engine that drives a generator producing electricity to drive electric motors in a similar concept to Diesel Locomotives.

[edit] Multiple Units vs. Locomotives

[edit] Advantages

Image:EN57 Units Poznan.jpg Image:GermanDMUcab.jpg There are several advantages of multiple units as compared to locomotive-hauled trains.

Energy Efficiency - MUs are more energy efficient than locomotive-hauled trains and more nimble, especially on grades, as much more of the entire train's weight (sometimes all of it) is placed on power-driven wheels, rather than suffer the dead weight of unpowered coaches;

No need to turn locomotive - MUs have cabs at each end, so that the train may be reversed without having to uncouple/re-couple and move the locomotive, which results in quicker turnaround times, reduced crewing costs, and enhancing safety This quicker turnaround time that results, and the reduced size (due to higher frequencies) compared with large locomotive-hauled trains, has made the MU a major part of suburban commuter rail services in many countries. MUs are also the type of train used by almost all underground railways.

Consist can be changed mid journey - MUs may usually be quickly made up or broken down into trains of varying lengths. Several multiple units may run as a single train, then be broken at a junction point into smaller trains for diverse destinations. Sometimes passage is available between the units, either for passengers or just for the train crew.

Reliability – Due to Multiple Unit trains 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 meaning the failure of this causes the train to be disabled (although some locomotive hauled trains may contain more than one power unit and 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 throughout the train from operating safely.

Axle load - because there is no locomotive with its heavy axle load, multiple units can venture onto 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 several axles, rather than just three or four.

[edit] Disadvantages

Multiple Units do have some disadvantages 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 break down, it is easy to replace it with a new one. 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 notably noisier than in a locomotive hauled train, due to the presence of underfloor machinery. This is particularly an issue with diesel powered MU's.

Obsolescence cycles - separating the motive power from the payload-hauling cars means that either can be replaced without affecting the other. At some times, locomotives have become obsolete when their cars are not, or vice versa.

It is difficult to have gangways between coupled sets, and an aerodynamic front end when leadings.

[edit] EMU features

Image:Class423inAachen.jpg It is not necessary for every single car in EMU/DMU 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 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. Simply speaking every end car can be used as a cab car whether it is motorised or motorless, if it is on an end of the train. This is the case in NJ Transit Arrows, Metro-North Railroad (New York) EMUs, etc. However, other EMUs can be driven/controlled only from 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 with high population densities. 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 network in India that provides commuter transport in all the major cities.

[edit] Commuter Rail

Image:Transperth-466-468-McIver-150705.jpg

Electric Multiple Units are popular on commuter and suburban rail networks around the world due to their fast acceleration, pollution free operation and quietness. Being quieter than DEMU, DMU and Locomotive drawn trains EMU's can operate later at night and more frequently without disturbing residents living near the railway lines. In addition tunnel design for EMU trains is simpler as provisions do not need to be made for diesel exhaust fumes.

In Australia EMU's are the primary means of commuter rail transport in all capital cities except for Adelaide and Hobart.

[edit] North America

Image:NJT-Arrow-Pantographs.JPG

Most long-distance trains in North America are locomotive-hauled, but commuter, subway, and light rail operations make extensive use of MUs. Most electrically powered trains are MUs, although there are some major exceptions: some Amtrak trains on the Northeast Corridor are drawn by electric locomotives; New Jersey Transit service on the same line is split between electric locomotives and EMUs.

M2,M4,M6 and future M8 EMUs which operate on New Haven Line of Metro-North Railroad, are dual mode meaning they can draw power from third rail by means of "shoes" on the trucks, or from overhead lines by means of pantographs. This feature is necessitated by the reliance on overhead lines between Pelham, NY and New Haven, CT, a section of track owned by Metro North but shared with Amtrak's Northeast Corridor service but switching back to third rail power 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, and many commuter routes are already electrified but also because of the difficulties posed by Federal Railway Administration rules limiting their use on shared passenger/freight corridors. DMUs are used on the River LINE 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 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.