Sixteen Class 121 single-car, double-ended 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".

The Dapol Model of the Class 121 and 122 include:

• Finely moulded body with many separately added fine details
• Accurately applied livery
• DCC ready
• DCC fitted models available
• Cast wheels with fine relief and appropriate colouring
• Tungsten chassis and all wheel pick up
• NEM pockets as standard
• Independently controllable and switchable directional lighting
• Dapol’s 5 Pole Skew wound Mega creep motor for smooth running and fine speed control
• Accessory bag with spares and other optional detail