Diesel multiple unit
A diesel multiple unit (DMU) is a train whose carriages have their own motors powered by a diesel engine.
Basics
DMU classification is subdivided into the form of the transmission: Diesel Mechanical Multiple Unit has a mechanical transmission; Diesel Hydraulic Multiple Unit has a hydraulic transmission; Diesel Electrical Multiple Unit has an electrical transmission.
The diesel engine may be overhead (very rare) or underslung. When underslung, it is also referred to as Railcar or, in North America, a Rail Diesel Car (RDC). The RDC was a single passenger car with two diesel engines and two sets of controls. Any number of cars could be connected together and all the propulsion systems controlled from a single operators station or cab. The other design places two or three cars together in a set. There are control systems at each end of the set and multiple sets can be connected together.
A regional rail train composed of DMU cars "scales" well (I.e. adding additional car sets automatically adds power). Distribution of the propulsion among the cars also results in a system that is less vulnerable to single-point-of-failure outages. Because they operate on diesel, there is no need to run overhead electric lines or electrified track, which can result in lower system construction costs.
More rugged than units strictly designed for operation on light rail, DMU systems can be safely operated within freight rail corridors, though safety regulations neccesitated by the use of heavier trains in North America generally prevent this, requiring that they be operated on separate tracks in North America.
Situation in the USA
 | The factual accuracy of part of this article is disputed. The dispute is about FRA rules. |
The US Federal Railway Administration effectively prohibits the use of lightweight DMUs on main line railways in the United States with higher buff strength requirements than European regulators. Several manufacturers, notably Colorado Railcar, are currently marketing DMUs suitable for mainline use in North America.
The term is also used in conjunction with individual diesel locomotives, especially in North America. In these cases the connections typically consist of seven air hoses and one electrical cable. The largest hose, located next to the coupler is the main air brake line or "train line". Outboard of this are two smaller hoses which link the air compressors on the locomotives. Further out are two hoses which control the brakes on the locomotives independently of the rest of the train. The two outermost hoses control the application of sand to the rails. An electrical cable above the coupler transmits throttle information from one locomotive to the next. Locomotives set up to use slugs have extra connections for transmitting electricity from the diesel electric generator on the "mother" unit to the traction motors on the slug.
The Los Angeles County Metropolitan Transit Authority approved an allocation of $250,000 for a feasibility study of DMUs for "future transportation options for the region" on July 5, 2006 (Ara Najarian, Metro Board Member).
Chicago's commuter rail line, Metra, is studying the use of DMUs on its newly proposed lines. They claim these DMUs will have better acceleration, be more fuel efficient, and seat more customers than the current diesel engine cab and double decker rail cars that are currently in use.
Amtrak is reportedly encouraging the state of Vermont to buy DMUs for Amtrak's state-subsidized Vermonter service, in order to save money over the current locomotive-pulled arrangement. [1]
Elsewhere in the world
Cityrail sydney
Manufacturers
DMU manufacturers include:
- Bombardier Transportation of Montreal, Canada
- Colorado Railcar
- Rotem of Seoul, South Korea
- Siemens Desiro DMU
- Integral Coach Factory of Chennai, India