train produced by RCF and
BHEL (
India) married pairs in
Port Chester,
New York 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. • A power car carries the necessary equipment to draw power from the
electrified infrastructure, such as
pickup shoes for
third rail systems and
pantographs for
overhead systems, and
transformers. • Motor cars carry the
traction motors to move the train, and are often combined with the power car to avoid high-voltage inter-car connections. • Driving cars are similar to a
cab car, containing a driver's cab for controlling the train. An EMU will usually have two driving cars at its outer ends. These can have
gangway connections to provide more operational flexibility, along with convenience for passengers. • Trailer cars are any cars (sometimes semi-permanently coupled) that carry little or no traction or power related equipment, and are similar to
passenger cars in a locomotive-hauled train. EMUs on the lines outside
Crewe Heritage Centre. Note the
gangway connection on the driving car. On third rail systems, the outer vehicles usually carry the pick up shoes with the motor vehicles receiving the current via
intra-unit connections. This helps to avoid '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 as
twin/"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, hauling
push-pull sets of trailer coaches. The
BR Class 432 was an example of this, hauling
TC trailer units on services on the
South West Main Line. ==High-speed EMUs==