British railcars and diesel multiple units

Diesel multiple units and railcars are trains, usually with passenger accommodation, that do not require a locomotive. Railcars can be single cars, while in multiple units cars are marshalled together with a driving position either end. As of December 2010^[update], 23 percent of the rail passenger cars used on Network Rail are part of an diesel multiple unit.

Some prototype steam-powered railcars appeared in the mid-19th century, and at the start of the 20th century over 100 were built. Diesel motors became powerful enough for a railway use after World War I, and the Great Western Railway built several single cars and multiple units. 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, and diesel electric multiple units were introduced.

In 1960 the Blue Pullman service was introduced using 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 diesel electric multiple unit, but for operational reasons the classification was changed and the power cars became identified as Class 43 locomotives.

From 1847–1849 William Bridges Adams built a number of steam railcars, vehicles with a steam engine for propulsion and passenger accommodation. These were the Express or Lilliputian, Fairfield and Enfield. Kitsons of Leeds built Ariel's Girdle in 1851. However, the next railcars were built in 1902 for the London & South Western Railway for the line from Fratton to East Southsea, although before entering passenger service it was lent to the Great Western Railway (GWR). By 1908 the GWR had purchased or built 99 railcars (or railmotors as they called them), but from 1917 began converting them into for use on push-pull trains with a steam locomotive, as these were more reliable and were able to haul additional carriages or goods wagons. Between 1905 and 1911 the Lancashire and Yorkshire Railway (L&YR) purchased or built seventeen steam rail cars, some running into the 1940s.

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