The British Rail Class 140 was the prototype of the Pacer diesel multiple unit.

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. Much of the bodywork was constructed using Leyland National bus components, with the exception of the cabs. 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. Throughout the 1980s, the sole member of the class functioned as a trials and demonstration unit, acting as a herald to the closely-related Class 141.

Since its withdrawal, the unit has been preserved at the Keith and Dufftown Railway.