A classic Belgian multiple unit of
The term multiple unit
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
, 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
unit operation was made possible by the development of multiple-unit train control by
the American inventor Franklin
. This allowed
-powered rapid transit
trains to be operated from a
single driving position.
successful test of an MU on a working rapid transit system was in
Chicago, on the South Side
"L", now part of the CTA Green Line.
Most MUs are powered either by a diesel
driving the wheels through a gearbox or hydraulic
), or by
, 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
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 another the
engine for head end power
an EMU might have one car carry the pantograph
, and another car carry the traction
Virtually all rapid transit
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 Japanese Shinkansen and the German ICE 3 high-speed
Multiple units versus locomotive-hauled trains
Multiple units have several advantages over locomotive-hauled
- 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
- 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
- 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
Multiple Units do have some disadvantages as compared to locomotive
- 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
- 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
- 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
- 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,
Well-known examples of MUs are the Japanese Shinkansen
and the last generation German
. Most trains in
Netherlands and Japan are MUs,
making them suitable for use in areas of high population
A new high-speed MU, the AGV
, was unveiled by
on February 5, 2008. It has a
claimed service speed of 360 km/h.
Ireland the railway operator Iarnród Éireann has purchased a
number of new DMUs since 1993 to replace older locomotives and
Japan, 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
. 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 Soviet or post-Soviet regional (mostly suburban) electrical multiple unit passenger
train. Elektrichkas are widespread in Russia, Ukraine and some
other countries of the former Soviet Union. The first
elektrichka ride occurred in August 1929 between Moscow and Mytishchi.
Ireland 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
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
families, and the brand
new Olympic Javelin
United States and Canada
Most long-distance trains in North America are locomotive-hauled.
However, commuters, rapid transit
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
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
. 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
EMUs are used on AMT
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
rules limiting their use on shared
passenger/freight corridors. When the Budd
was developed following World War II, it was adopted for
many secondary passenger routes in the United States (especially on
the Boston and Maine
) 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
Mahalat on Vancouver Island.
used on the RiverLINE in New Jersey.
Currently Colorado Railcar
is demonstrating an FRA
Crash Compliant DMU in the United States. NJ
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
Haven 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