British Rail Class 121
 Sixteen Class 121 single-car driving motor vehicles were built from 1960 numbered 55020–55035. These were supplemented by ten trailer vehicles numbered 56280–56289 (later renumbered 54280–54289). They had 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 now endorsed by current operator Chiltern Railways).