The Class 121 and 122 Bubble cars form part of the first generation of DMUs produced by British Rail to operate on branch lines and secondary routes. The Class 121 was built by Pressed Steel in 1960, 15 single power cars numbered W55020 to W55035 and 10 trailer cars numbered W56280 to W56289 were manufactured. Until recently there were still two 121s in service and eight are preserved.

Features of the Class 121 DMU includes:
• Heavy Diecast chassis for added weight and superior tractive effort
• Dual motorised power bogies with all wheel pickup
• DCC & DCC Sound Fitted options available
• Lowered flooring to minimise the intrusion of the DCC speaker etc inside the model
• Headcode and destination boxes with removable glazing permitting number to be mounted behind glazing.
• Separate grab handles on passenger doors
• Separate etched grills for guards windows
• Guards area with separate lighting
• Exquisite passenger compartment detailing
• High level of detailing on the bogies and underframe of the locomotive
• 2 buffer types
• Two sides with high passenger grab handles and low passenger grab handles
• Two roof types (large box and small box types)
• Provision for 3 chassis types (additional side steps and varying underframe detail and flat, single driver ended design)
• Switches accessible either by easily removable roof or from underneath the model.
• Independently controllable cab, passenger compartment, guard compartment and directional lighting—easy access switches for controlling lights in the DC model
• Multi speaker factory fitted sound system on board (optional)