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 a 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 railway use after World War I, and 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, 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.