In 1971, British Rail Engineering Limited (BREL) introduced a new family of electric multiple units for local transport with the prototypes of the Classes 445 and 446, which became known under the type code “PEP”. The prototypes included one two-car and two four-car sets, in which all four axles of all cars were powered. Closely spaced seats with low backrests and many electrically operated doors were used for local transport. Also new were automatic couplings, which were based on the American design and additionally had electrical and pneumatic connections. The trains could thus be coupled or uncoupled during operation without the driver having to get out or requiring assistance.
Between 1973 and 1977, these three trains were subjected to extensive trial operations, during which they were mostly used coupled together to form a ten-car unit. They were then used by the network operator until 1983 and were also used to test new assemblies such as bogies. From 1976, the first production trainsets based on the prototypes were built, with the similar classes 313, 314 and 315 making the start. What these three classes had in common was that they consisted of powered control cars and non-powered trailers and were designed for operation under 25,000 V AC. The pantograph was located on the center car to better distribute the equipment over the train.
South Western Railway 455/7 at Clapham Junction station in September 2018
Smiley.toerist 
The first was the class 313, which was built from February 1976 and additionally had contact shoes for operation under 750 V direct current via the third rail. It was used on the former Great Northern routes between London and Hertfordshire, partly using Underground tunnels. Although these tunnels were widened to accommodate regular trains, the 313 got a lower roofline.
This was followed by the class 314, which was built from 1979 for the Glasgow area and, like the class 313, was made up of three cars each. From 1980, the class 315 followed, which had a second trailer and thus had four cars each. In the course of their career, the trains were used in other areas. The contact shoes were removed from some examples of class 313 because they were no longer needed. After modernization in the areas of electronics, lighting and accessibility, they were retired from 2018. So the classes 313, 314 and 315 were replaced by the classes 717, 385 and 710.
From 1978 the Class 507 and 508 were developed for use on the Merseyrail network around Liverpool. There, the power supply is provided by direct current via the third rail. The difference between the classes is that the 507 class has only one trailer between the two powered control cars, while the 508 class has two trailers. Immediately after delivery, the 508 was transferred to London Waterloo for temporary use until the Class 455 was available in sufficient numbers. For use in the Merseyrail network, a trailer was removed again so that both classes were almost identical again. The Class 507, on the other hand, stayed on the Merseyrail network throughout its life. From 2019, both were replaced by Stadler's class 777 “Metro”.