The PEP Stock were prototype electric multiple units used on British Rail's Southern and Scottish Regions during the 1970s and early 1980s. They were forerunners of the British Rail Second Generation electric multiple unit fleet. Three units were built, one two-car unit (2001), and two four-car units (4001/4002). Under TOPS, the driving cars were originally classified as Class 461 with the non-driving motor cars as Class 462. They were later reclassified as Class 445 (4PEP) and Class 446 (2PEP). Internal layout was for commuter services; low-backed, bus-style 2+2 seating in open saloons, wide gangways with hanging straps, and no lavatory facilities. They were the first electric multiple units designed by British Rail with electric sliding doors, outside the Scottish Region. They were unable to operate with any other stock due to their new coupling system. Externally, 2001 was finished in unpainted aluminium, while 4001/4002 were painted in all-over Rail Blue. In passenger use, they normally operated together as a ten-car formation. Design On the 4-PEP units the non-driving cars were powered, similar to London Underground practice at the time, but a departure from previous British Railways designs where there had not been any 4-car units with all axles powered. Also introduced for the first time was rheostatic braking with cam-controlled analogue logic, and fluorescent lighting working at 50 Hz. Interior design was open saloons with 2+2 seating, although this was not considered to be finalised until after trial running, and also included communicating doors between coaches and between units, with the driving cars having 2 pairs of sliding doors each and 3 pairs on the non-driving coaches. Passenger use All three units were initially assigned to the South Western Division of the Southern Region, based at Wimbledon Park and Strawberry Hill. Upon arrival, they were used under an extensive testing programme prior to entering full passenger service in June 1973, on services from Waterloo to , and . In August 1973, all three units were moved to the South Eastern Division, where they worked services from Cannon Street/ to , and . Although capable of being used as a single 10-car formation on the SED, a number of failures saw them returned to the SWD after a month. Departmental use After the units were withdrawn from passenger service, they continued to be used by the Research department for further tests. For this purpose, the units and individual carriages were all