British railcars and diesel multiple units

Diesel multiple units and railcars are trains, usually with passenger accommodation, that are self-powered with diesel engines and do not require a locomotive. Railcars can be single carriages while, in multiple units, cars are marshalled together with a driving position at either end. As of December 2010, 23% of the rail passenger cars used on the National Rail network were part of a diesel multiple unit (DMU).

Some prototype steam-powered railcars appeared in the mid-19th century and over 100 were built at the start of the 20th century. Diesel motors became powerful enough for railway use after World War I; the Great Western Railway built several single cars and multiple units in the 1930s, which lasted until the 1960s. A 1952 report recommended the trialling of lightweight diesel multiple units, followed by plans in the 1955 Modernisation Plan for up to 4,600 diesel railcars. Most of these had a mechanical transmission, but the Southern Region had experience of DC electric multiple units (EMU) and diesel-electric multiple units (DEMU) were introduced.

In 1960, the Blue Pullman service was introduced as a high-speed trainset, consisting of coaches sandwiched between two power cars. This arrangement was later used for the InterCity 125, permitting a top speed of 125 miles per hour (201 km/h). Initially this train was considered to be a DEMU but, for operational reasons, the classification was changed and the power cars became identified as Class 43 diesel locomotives.