Derived from the Pressed Steel class 117 units the single car class 121 unit was a Driving Motor Brake Second (DMBS) car with a second driving cab at what would otherwise be the inner or luggage van end. 16 cars were built, primarily for the Western region, operating a number of branch lines along the Thames valley, in the Bristol area and throughout Devon and Cornwall. A number of single-cab trailer cars were also built, using the same bodyshell as the class 117 DMS, which allowed the train capacity to be increased when needed. The class 121 units featured the standard BR Blue Square multiple control system, so class 121 cars could be added to other DMU trains. A DMBS could be used as a stand in for a motor car on a three-car set and in later years the trailers were often paired with motor cars from other DMU classes to make 2-car units.
The class 121 units were put through the refurbishment programme in the early 1980s, receiving new interiors and flourescent lighting along with the brighter blue and grey livery. Cars operating in the London and Thames Valley areas joined Network South East on sectorisation, while withdrawn cars were often put to new uses for driver training and route learning, video infrastructure surveying and Sandite spraying (Sandite is an adhesion-improving substance). Two units are still in service with Chiltern Railways, one with Network rail and eight have been preserved.