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Anelectric multiple unit orEMU is amultiple-unittrain consisting of self-propelled carriages usingelectricity as the motive power. An EMU requires no separatelocomotive, as electrictraction motors are incorporated within one or a number of the carriages. An EMU is usually formed of two or more semi-permanently coupled carriages. However, electrically powered single-unitrailcars are also generally classed as EMUs. The vast majority of EMUs are passenger trains but versions also exist for carrying mail.
EMUs are popular on intercity, commuter, and suburban rail networks around the world due to their fast acceleration and pollution-free operation,[1] and are used on most rapid-transit systems. Being quieter thandiesel multiple units (DMUs) andlocomotive-hauled trains, EMUs can operate later at night and more frequently without disturbing nearby residents. In addition, tunnel design for EMU trains is simpler as no provision is needed for exhausting fumes, although retrofitting existing limited-clearance tunnels to accommodate the extra equipment needed to transmit electric power to the train can be difficult.
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Multiple unit train control was first used in the 1890s.
TheLiverpool Overhead Railway opened in 1893 with two-car electric multiple units,[2] controllers in cabs at both ends directly controlling the traction current to motors on both cars.[3]
The multiple unit traction control system was developed byFrank Sprague and first applied and tested on theSouth Side Elevated Railroad (now part of theChicago 'L') 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 of the traction motors in the train are controlled in unison.
As technology improved with more compact and reliable electrical systems becoming available, EMUs became more common and supplanted locomotive hauled stock on many networks. This process was accelerated on crowded networks with frequent trains, as the operational advantages in using EMUs outweighed the initial cost.
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The cars that form a complete EMU set can usually be separated by function into four types: power car, motor car, driving car, and trailer car. Each car can have more than one function, such as a motor-driving car or power-driving car.
On third rail systems, the outer vehicles usually carry the pick up shoes with the motor vehicles receiving the current viaintra-unit connections. This helps avoiding 'gapping' events where the unit is not in contact with the third rail and needs rescuing. For modern EMUs that operate on AC overhead systems, the traction motors have often moved from the power car to separate motor cars. The power car retains the transformer and sends the required energy via connectors to the motor cars. This helps to distribute weight along the length of the EMU and reduces the maximum axle load and track access/maintenance costs. This is not a consideration with DC powered sets as no transformer is required and any other conversion equipment is lighter.
The majority of EMUs are set up astwin/"married pair" units or longer sets. In addition to the traction motors, the ancillary equipment (air compressor and tanks, batteries and charging equipment, traction power and control equipment, etc.) are shared between the cars in the set. Since no car can operate independently, such sets are only split at maintenance facilities. For longer length EMUs (8+ cars) the unit will often have duplicate power, traction & braking systems in two halves of the set, providing redundancy for increased weight and cost.
Advantages of married pair or longer sets include weight and cost savings over single-unit cars (due to reducing the ancillary equipment required per set) while allowing multiple cars to be powered, unlike a motor-trailer combination. Each EMU has only two control cabs, located at the outer ends of the set. This saves space and expense over a cab at both ends of each car and provides more capacity. Disadvantages include a loss of operational flexibility, as trains must be multiples of a set length, and a failure on a single car could force removing the entire set from service.
In rare circumstances EMUs can operate like locomotives, haulingpush-pull sets of trailer coaches. TheBR class 432 was an example of this, haulingTC trailer units on services on theSouth West Main Line.
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Some of the more famous electric multiple units in the world are high-speed trains, including the:
EMUs powered byfuel cells are under development. If successful, this would avoid the need for anoverhead line orthird rail. An example isAlstom’shydrogen-poweredCoradia iLint.[4] The termhydrail has been coined for hydrogen-powered rail vehicles.[citation needed]
Manybattery electric multiple units are in operation around the world, with the take up being strong. Many are bi-modal taking energy from onboard battery banks and line pickups such as overhead wires or third rail. In most cases the batteries are charged via the electric pickup when operating on electric mode.[citation needed]
EMUs, when compared withelectric locomotives, offer:[5]
Electric locomotives, when compared to EMUs, offer:
This is one of the original motor coaches which has electric motors mounted beneath the floor, a driving cab at one end and third class accommodation with wooden seats.
Media related toElectric multiple units, motor coaches and railcars at Wikimedia Commons