Multiple unit
|
A classic Belgian multiple unit of type 74 |
A
multiple unit (MU) is a passenger
train whose
carriages have their own
motors, either
diesel ("
DMUs") or
electric ("
EMUs"), and can be coupled with other similar units to operate together, in multiple.
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.
There are several advantages of multiple units as compared to locomotive-hauled trains.
*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;
|
cabs which are both by the head and tail of German DMU |
*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;
*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.
The quicker turnaround time that results, and the reduced size 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.
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.
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.
|
Class 423 German EMU train in Aachen-HBF |
In some EMU trains, every car is equipped with a driving console, and other controls necessary to operate the train. Simply speaking every 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.
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.
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. EMUs are used on
AMT's
Montreal/Deux-Montagnes line.
*
Push-pull (mode of operation for locomotive-hauled trains)
*
Rail terminology*
Multiple-unit train control*
Pacer (train) units.
*
Railcar
