The British Rail Class 121 is a single-car double-ended diesel multiple unit. 16 driving motor vehicles were built from 1960, numbered 55020–55035. These were supplemented by ten single-ended trailer vehicles, numbered 56280–56289 (later renumbered 54280–54289). They have a top speed of 70 mph with slam-doors and vacuum brakes. The driving motor vehicles were nicknamed "Bubble Cars" by some enthusiasts (a nickname endorsed and made official by final passenger service operator Chiltern Railways).
The Class 121 vehicles were introduced in 1960 for use on the Western Region of British Rail. They were used on various lightly used branch lines in Cornwall including the Looe branch, the branch lines off the main line in the Thames Valley including the Greenford Branch Line, the Bridport branch line (closed 1975), and the Severn Beach line in Bristol. In 1978 all of the units were still allocated to Western Region depots. The Class 121 is Britain's longest serving DMU, operating in passenger service for 57 years until 2017.