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

Railfan

A railfan or rail buff , railway enthusiast or railway buff , or trainspotter , is a person interested in a recreational capacity in rail transport...

 (a nickname now endorsed by current operator Chiltern Railways

Chiltern Railways

Chiltern Railways is a British train operating company. It was set up at the privatisation of British Rail in 1996, and operates local passenger trains from Marylebone station in London to Aylesbury and main-line trains on the Chiltern Main Line to Birmingham Snow Hill with its associated branches...

).
The Class 121 were built for use on the Western Region of British Rail.