Shildon since March 2020. Multiple units based on the Mark 3 The Mark 3 design proved to be highly adaptable and formed the basis of BR's Second Generation multiple unit fleet, introduced from the early 1980s, with the following classes having Mark-3-based bodyshells: DMU Classes Class 150 Class 210 AC EMU Classes Class 317 Class 318 Class 319 Class 320 Class 321 Class 322 Class 325 DC EMU Classes Class 442 Class 455 Class 456 Bi-mode EDMU Classes Class 768 Class 769 Class 799 The cars for Classes 150, 210, 317, 318, 319, 320, 321, 322, 325, 455 and 456 units are built to a shorter 20 m 'suburban' design. Those for Class 442 are 23 m and are very similar to the Mark 3 coach. The main visual difference is the swing plug automatic doors rather than the traditional manual "slam-doors". The Class 153 and Class 155, while of the "Sprinter family", are built by British Leyland and have no connection with the Mark 3, neither does the Class 156, built by Metro-Cammell. The final batch of "Sprinters" of Class 158 (some rebuilt as Class 159) are of a different design built from aluminium extrusions. Its design being intermediate between that of the Mark 3 and the Networker (train). Nine 450 Class DMUs were built at Derby Litchurch Lane Works for Northern Ireland Railways using Mark 3 bodyshells and Mark 1 underframes, together with refurbished power units and traction motors, recovered from the former Class 70 units. The last Mark-3-based EMUs built are the Class 325 EMUs built for Royal Mail in 1995. Mark 3 coaches overseas Ireland The Republic of Ireland's national rail operator, Iarnród Éireann, ordered Mark 3 carriages built between 1980 and 1989, with bogies for the Irish gauge of 1600 mm (5 ft 3 in). The fleet consisted of 124 Mark 3 and nine Mark 3A Internationals, which worked only the Dublin–Galway service, branded or Hound of the Seas as it was a coast-to-coast route. Throughout the 1990s and 2000s they were the backbone of the Irish InterCity rolling stock. They were built with automatic plug doors, which initially caused some concern as additional time and resources were required to perfect them. The design was later used on the Class 442 "Wessex Electrics". Most of the fleet was air-conditioned, except for a small number of coaches built as outer-suburban stock, which ran in push-pull configuration.