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.

977723 started out life as 55021 under BR. In 1994 the unit was photographed wearing the Network South East livery with its departmental number of 977723 where it served as a route learning unit as well as a sandite application vehicle. The locomotive would wear the Railtrack livery This Railroad model features a simplified running mechanism including a three pole motor and more of a direct drive to the motor, as such it is an ideal starter model. The model has an 8 pin DCC socket for those who which to run the model on a digital layout and a more robust body is the perfect way to avoid damage to the model as a beginner.