A Diesel Multiple Unit (DMU) is a diesel powered train of semi permanently connected powered cars. Most come in two – four car sets. In its simplest two-car form; the first car has the diesel motor in it, and the second car is a trailer (Non-powered). A widely used three-car form; the first and the last cars have diesel engines, and the center car is the trailer (Non-powered) car.
The DMU's origins can be traced back to the steam trams working in Britain on the London, Midland & Scotland (LMS) and London & North Eastern Railways in the 1920s and '30s. Great Western Railway (GWR) introduced diesel rail-cars in 1933, for branch-line duties. But about a year later GWR was running a express service between Birmingham (England) and Cardiff (Wales) 100 miles (160 km) away. These three-car sets had a non-powered dining car in the middle. Hence; the service was aimed at businessmen. These rail-cars were just anticipating the true DMUs to come.
In Britain it wasn't until the 1950s that the rail-car truly became the DMU, and it was the 1955 Modernization Plan that caused it. They soon took over the stopping services from steam haulage. Under the powered cars, hung direct drive diesel engines (albeit through a transmission). These DMUs are retrospectively called Heritage DMUs today.
Still in Britain; the Second Generation of DMUs was introduced in the mid-1980s. Referred to as the Sprinter series, they had hydraulic torque-converters (transmissions), sliding doors and offered far superior ride quality over the Heritage DMUs. Another series, the Pacer; was a smaller and lighter four wheeled type. But the lack of trucks (bogies) made the ride quality poor. Throughout the United Kingdom, the DMU has mostly replaced the locomotive hauled trains on short to medium routes.
- Book: The Complete Book of Locomotives written by Colin Garratt and Published by Hermes House. ISBN: 978-1-84477-022-9.