In order to modernize passenger transport over short distances, British Railways approached the industry in 1954, which was to produce a large number of light diesel railcars. Since there was no standard design due to lack of time, several companies had a relatively free hand in the development and delivered different models, which, however, had many things in common.
All DMUs consisted of four-axle cars, which were usually coupled to form combinations of two to six vehicles. These combinations had only one cab at each end, there were only a few one-car vehicles with two cabs. The individual cars were either motorized with two diesel engines each, each of which drove the inner axle of a bogie, or unmotorized.
Most of the railcars were powered by British United Traction (BUT) diesel engines, each with 150 hp. This was a cooperation between AEC and Leyland, which also manufactured trolleybuses in addition to diesel engines for rail vehicles. The power was transmitted over a mechanical four-speed gearbox. Some classes had more powerful engines from BUT, Leyland or Rolls-Royce with up to 237 hp, some of which worked on a hydraulic gearbox. The maximum speed was 70 mph for all variants.
121032 “Bubble Car” operated by Arriva Trains Wales in June 2019 at Cardiff Queen Street
Hugh Llewelyn 
A total of more than 3,400 cars came from a total of ten manufacturers. The most successful variant was Metropolitan Cammell's Class 101, which was used in combinations of two to four cars. Combinations of two cars were made up of either a power car and a driving trailer or two railcars and were then referred to as “Power Twins”. Metro-Cammell supplied nearly 700 Class 101, 102 and 111 cars.
More than 1,100 came from the British Railways' works in Derby. The largest numbers of these were produced of the two- to four-car class 108 and the two- to three-car class 116. The class 116 was one of the variants that were designed for suburban traffic and had doors in each row of seats, which in Great Britain was also known as the “Slam Door Train”. A further 400 examples came from the BR workshops in Swindon. This also included the class 124, which consisted of six cars each, four of which had two 231 hp engines and thus achieved good acceleration.
Gloucester Railway Carriage and Wagon Company supplied just over 200 and Cravens just over 300 carriages. The Class 128 from Gloucester and the Class 129 from Cravens were parcel DMUs which had a parcel compartment without windows instead of a passenger compartment. While the 129 class had the standard 150 hp engines, the 128 class had two Leyland Albion engines, each with 231 hp.
Class 128 parcel DMU in August 1982 at Manchester-Victoria
Hugh Llewelyn Another notable number of more than 400 came from the Birmingham Railway Carriage and Wagon Company. These consisted of 302 cars for the two- to four-car Class 104, Class 118 and Class 110, the latter with two Rolls-Royce engines of 180 hp. Smaller numbers came from Park Royal, Pressed Steel and D. Wickham & Co.
The large number and variety of classes ensured that the first generation diesel multiple units were used in all parts of the country. Production of the last pieces was already completed in 1963. In the 1980s, the Pacer and Sprinter railcars were developed, which were intended to completely replace the vehicles of the first generation. Since the Pacer was not quite convincing and the number of Sprinters was not quite sufficient, not all railcars of the first generation could be retired in the 1980s. Some examples of class 101 could not be retired until 2003, while the one-car vehicles of class 121 “Bubble Car” were in regular use until 2017.