Multiple unit


A multiple-unit train is a self-propelled train composed of one or more carriages joined, and where one or more of the carriages have the means of propulsion built in. By contrast, a locomotive-hauled train has all of the carriages unpowered.
An implication of this is that all the powered carriages need to be controllable by a single engineer or driver, which is a case of the broader concept of multiple-unit train control. In other words, all "multiple units" employ some variation of multiple-unit train control. In the broader context "unit" means any powered rail vehicle, including locomotives and powered cargo-carrying carriages. In the context of this article, "unit" refers specifically to the latter only.
What follows is that if coupled to another multiple unit, all MUs can still be controlled by the single driver, with multiple-unit train control.
Although multiple units consist of several carriages, single self-propelled carriages – also called railcars, rail motor coaches or railbuses – are in fact multiple units when two or more of them are working connected through multiple-unit train control.

History

Multiple-unit train control was first used in electric multiple units in the 1890s.
The Liverpool Overhead Railway opened in 1893 with two-car electric multiple units, controllers in cabs at both ends directly controlling the traction current to motors on both cars.
The multiple-unit traction control system was developed by Frank Sprague and first applied and tested on the South Side Elevated Railroad in 1897. In 1895, derived from his company's invention and production of direct-current elevator control systems, Frank Sprague invented a multiple-unit controller for electric train operation. This accelerated the construction of electric-traction railways and trolley systems worldwide. Each car of the train has its own traction motors: by means of motor control relays in each car energized by train-line wires from the front car, all the traction motors in the train are controlled in unison.

Design

Most MUs are powered either by traction motors, receiving their power through a third rail or overhead wire, or by a diesel engine driving a generator producing electricity to drive traction motors.
A MU has the same power and traction components as a locomotive, but instead of the components being concentrated in one car, they are spread throughout the cars that make up the unit. In many cases these cars can only propel themselves when they are part of the unit, so they are semi-permanently coupled. For example, in a DMU one car might 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.
MU cars can be a motor or trailer car, it is not necessary for every one to be motorized. Trailer cars can contain supplementary equipment such as air compressors, batteries, etc.; they may also be fitted with a driving cab.
In most cases, MU trains can only be driven/controlled from dedicated cab cars. However, 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. An example of this arrangement is the NJ Transit Arrows.

Weight reduction

An advantage of multiple unit trains is that they can be engineered to be lighter than locomotive-hauled trains with separate carriages, using lightweighting techniques to reduce energy use, track wear, and operating costs. The term "light-weight train" was first used in the 1930s, with early designs such as the 1934 M-10000 and Pioneer Zephyr in the United States, and Germany's 1932 Flying Hamburger.
Modern light-weight multiple units typically use fewer bogies and distribute traction equipment across the train, improving efficiency and axle loading. However, they require particular design attention for bridge loading and crosswind safety, and often include noise and vibration mitigation systems.
Examples from the 21st century include the lightweight Talgo sets used in Spain and exported to Germany and Denmark, India's modern Vande Bharat Express units, and the upcoming TELLi fleet for SNCF regional lines in France.

Passenger multiple units

Virtually all rapid-transit rolling stock, such as on the New York City Subway, the London Underground, the Paris Metro and other subway systems, are multiple-units, usually EMUs. Most trains in the Netherlands and Japan are MUs, being suitable for use in areas of high population density.
Many high-speed rail trains are also multiple-units, such as the Japanese Shinkansen and the German Intercity-Express ICE 3 high-speed trains. A new high-speed MU, the AGV, was unveiled by France's Alstom on 5 February 2008. It has a claimed service speed of. India's ICF announced the country's first high-speed engine-less train named 'train 18', which would run at maximum speed.
Passenger multiple units can be divided into articulated trains and non-articulated trains. The first type can be divided again into the TGV/AGV subtype and the Talgo subtype.

Freight multiple units

For freight traffic powered through multiple locomotive "units", see Multiple-unit train control and Distributed power.
Multiple units have been occasionally used 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.

Comparison to locomotive-hauled trains

Advantages

Energy efficiency

They are more energy-efficient than locomotive-hauled trains.

Gradients

They have better adhesion, as more of the train's weight is carried on driven wheels, rather than the locomotive having to haul the dead weight of unpowered coaches.

Acceleration

They have a higher power-to-weight-ratio than a locomotive-hauled train since they don't have a heavy locomotive that does not itself carry passengers, but contributes to the total weight of the train. This is particularly important where train services make frequent stops, since the energy consumed for accelerating the train increases significantly with an increase in weight. Because of the energy efficiency and higher adhesive-weight-to-total-weight ratio values, they generally have higher acceleration ability than locomotive-type trains and are favored in urban trains and metro systems for frequent start/stop routines.

Turnaround times

Most of them have cabs at both ends, resulting in quicker turnaround times, reduced crewing costs, and enhanced safety. The faster turnaround time and the reduced size 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. However, the need to turn a locomotive is no longer a problem for locomotive-hauled trains due to the increasing use of push pull trains.

Failure

Multiple units 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 shorter trains for different destinations. As there are multiple engines/motors, the failure of one engine does not prevent the train from continuing its journey. A locomotive-drawn train typically has only one power unit, whose failure will disable the train. However, 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.

Axle loads

They have lighter axle loads, allowing operation on lighter tracks, where locomotives may be 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. They generally have rigid couplers instead of the flexible ones often used on locomotive-hauled trains. That means brakes/throttle can be more quickly applied without an excessive amount of jerk experienced in passenger coaches. In a locomotive-hauled train, if the number of cars is changed to meet the demand, acceleration and braking performance will also change. This calls for performance calculations to be done taking the heaviest train composition into account. This may sometimes cause some trains in off-peak periods to be overpowered with respect to the required performance. When 2 or more multiple units are coupled, train performance remains almost unchanged. However, in locomotive-hauled train compositions, using more powerful locomotives when a train is longer can solve this problem.

Disadvantages

Maintenance

It may be easier to maintain one locomotive than many self-propelled cars. 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 casualties than one with a locomotive.

Failure

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. However, if the train consists of more than one multiple unit they are often designed such that in the event of the failure of one unit others in the train can tow it in neutral if brakes and other safety systems are operational.

Idle trains

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 to use in passenger transportation.