A multiple-unit train (or multiple unit (MU)) is a self-propelled train or trainset 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 entirely unpowered carriages.
An implication of this is that all powered carriages must be controllable by a single engineer or driver, which falls under 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 (that do not carry cargo) and powered cargo-carrying carriages. In the context of this article, "unit" refers specifically to the latter only (whether the cargo is passengers or some other cargo).
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 (regardless of whether passengers can walk between the units or not).
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 (now part of the Chicago 'L') in 1897. In 1895, Frank Sprague, whose company invented and produced direct-current elevator control systems, developed 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. Motor control relays in each car, energized by train-line wires from the front car, control all the traction motors in the train in unison.
Design
Most MUs are powered either by traction motors, receiving their power through a third rail or overhead wire (EMU), or by a diesel engine (DMU) 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 motor or trailer cars; not all need to be motorized. Trailer cars can carry supplementary equipment such as air compressors and batteries, and 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; every car can be used as a cab car, whether it is motorized or not, at 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, is 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."France unveils super-fast train", BBC News, 5 February 2008. Retrieved 5 February 2008.
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 (which uses Jacobs bogies) and the Talgo subtype.
Freight multiple units
Multiple units have occasionally been used for freight traffic, such as carrying containers, or for maintenance trains. The Japanese M250 series train has four front and end carriages that are EMUs, and has been operating since March 2004. The German CargoSprinter has 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 because they don't have a heavy locomotive that carries no passengers and adds to the train's total weight. This is particularly important where train services make frequent stops, since the energy required to accelerate the train increases significantly with 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 (due to higher frequencies) as compared to large locomotive-hauled trains have made the MU a major part of suburban commuter rail services in many countries. Most rapid transit systems also use MUs. 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, and the failure of that unit will shut down 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 power unit.
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 distributed across many axles rather than just the four or six of a locomotive. They generally have rigid couplers rather than the flexible ones commonly used on locomotive-hauled trains. That means brakes/throttle can be applied more quickly without the excessive jerk experienced in passenger coaches. In a locomotive-hauled train, adjusting the number of cars to meet demand will also affect acceleration and braking performance. This calls for performance calculations to be done, taking the heaviest train composition into account. This may sometimes cause some trains during off-peak periods to be overpowered relative 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 for longer trains 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 (where the heavy locomotive would act as a "crumple zone").
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 train will often require a whole new train and time-consuming switching operations; passengers would also be asked to evacuate the failed train and board another. However, if the train consists of more than one multiple unit, they are often designed so that, in the event of a failure of one unit, the others in the train can tow it in neutral if the brakes and other safety systems are operational.
Idle trains Idle trains do not waste expensive motive power resources. Separate locomotives mean that costly motive power assets can be moved around as needed and used to haul freight trains. A multiple-unit arrangement would limit the use of these costly motive power resources to passenger transportation.
Passage between units It is difficult to have gangway connections between coupled units while retaining an aerodynamic leading front end. Because of this, there is usually no passage between high-speed coupled units, though lower-speed coupled units frequently have connections between coupled units. This may require more crew members, so that ticket inspectors, for example, can be present in all of them. This leads to higher operating costs and lower use of crew resources. In a locomotive-hauled train, one crew can serve the train regardless of the number of cars in the train, provided the limits of individual workload are not exceeded. Likewise, in such instances, buffet cars and other shared passenger facilities may need to be duplicated in each unit, reducing efficiency.
Flexibility Large locomotives can be used instead of small locomotives where 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-drawn train. This is not so easy for a multiple-unit system, 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 the number of cars. Cars can be added or removed one by one, but on multiple units, two or more units must be coupled. This is not so flexible.
Noise The passenger environment in a multiple unit is often noticeably noisier than in a locomotive-hauled train, due to the presence of underfloor machinery. The same applies to vibration. This is a particular problem with DMUs.
Obsolescence