British Rail's third design of carriages was designated Mark 3. There were three sub-divisions; Mark 3, Mark 3A and Mark 3B. Mark 3 refers solely to carriages used as part of High Speed Train sets, while Mark 3A and 3B carriages are both locomotive-hauled stock varieties, originally used on the West Coast Main Line route from London Euston. Locomotive-hauled trains are now being replace on the WCML by new Class 390 electrical multiple units. Some Mark 3 carriages are being transferred to 'One' Anglia to replace their current Mark 2E/2F fleet.

Ireland's national rail operator, Iarnród Éireann have a fleet of these BR carriages, configured with 1600 mm (5 ft 3 in) gauge bogies. As of 2004 they form the main body of the intercity rolling stock on the Irish railway network. These models were built during the 1980s – in 1980, 1986 and 1989. They are all air-conditioned with automatic doors - the doors caused some concern at the time as additional time and resources was required to perfect them. Some currently run in push-pull configuration and some have windows which open. A number of the units are first class, there are several dining carriages and some driving van trailers (DVTs include passenger seating). An accompanying generator van supplies power. Iarnród Éireann intend to replace these carriages within the next few years, replacing them with new carriage built by CAF of Spain, and "regional railcars" (DMUs).