The British Rail Class 313 is a dual-voltage electric multiple unit (EMU) train built by British Rail Engineering Limited's Holgate Road carriage works between February 1976 and April 1977. They were the first production units that were derived from British Rail's 1971 prototype suburban EMU design which, as the BREL 1972 family, eventually encompassed 755 vehicles over five production classes (313, 314, 315, 507 and 508). They were the first second-generation EMUs to be constructed for British Rail and the first British Rail units with both a pantograph for overhead lines and contact shoe equipment for supply. They were, additionally, the first units in Britain to employ multi-function automatic Tightlock couplers, which include electrical and pneumatic connections allowing the coupling and uncoupling of units to be performed unassisted by the driver whilst in the cab. The Class 313 units are the oldest EMUs on the National Rail network, at 46 years old; they entered service in 1976 However, the even-older 1972 Stock and 1973 Stock are still in service on London Underground. Description The Class 313 was developed following extensive trials with the prototype Class 445 "PEP" built in the early 1970s. The 313 is similar to the Class 314 (Glasgow), Class 315 (Anglia suburban — east London), Class 507 (Merseyside) and Class 508 (Merseyside, formerly Southern Region). The Class 313 directly replaced the Class 105 and 106 diesel multiple units previously on the Great Northern Inner Suburban services between London and Hertfordshire. Since they were designed for use on Great Northern Inner Suburban services from to , , and , which included a section of 'tube' line built to take standard size trains between and Moorgate, they are built to a slightly smaller loading gauge than conventional trains. They are standard length and width, but the roof is lower, most noticeable due to the lack of a "well" for the Stone Faiveley AMBR pantograph on the centre coach. They have to comply with regulations for underground trains, such as having doors at each end of the train for evacuation onto the tracks, and when on 750 V DC supply the traction supply for each motor coach is separate, whereas on conventional 750 V DC trains each coach in a unit is linked by a 750 V bus line. Due to this, each motor coach has shoe gear on both bogies, whereas normally it would only be on the leading