It was constructed between 1979 and 1981 in response to a desire within British Rail to develop a capable railbus for its smaller branch line services.

Based on the single car railbus prototypes, the Class 140 was built to BR's then stringent regulations regarding crashworthiness and resistance to end loading; as such, much of its intention lightweight 'bus on a wagon' look was lost, becoming a more substantial vehicle.

By the beginning of the 1980s, British Rail (BR) operated a large fleet of first generation DMUs, which had been constructed in prior decades to various designs.

[6] While formulating its long-term strategy for this sector of its operations, British Rail planners recognised that there would be considerable costs incurred by undertaking refurbishment programmes necessary for the continued use of these aging multiple units, particularly due to the necessity of handling and removing hazardous materials such as asbestos.

[7] In the concept stage, two separate approaches were devised, one involving a so-called railbus that prioritised the minimisation of both initial (procurement) and ongoing (maintenance and operational) costs, while the second was a more substantial DMU that could deliver superior performance than the existing fleet, particularly when it came to long-distance services.

[7] While the more ambitious latter requirement would ultimately lead to the development of the British Rail Class 151 and the wider Sprinter family of DMUs, BR officials recognised that a cheaper unit was desirable for service on the smaller branch lines that would not be unduly impacted by lower performance specs or a high density configuration.

[7] Being built as an initial prototype, it was determined that while this design was workable and had delivered some positive attributes, such as a favourable level of isolation to both noise and vibration, it also incurred a considerably high weight cost.

[3] Initial testing with the Class 140 uncovered an issue with its detectability by track circuits, this was reliable resolved by swapping the material of the brake blocks from a composite to iron.