Originally conceived as a cheaper, more reliable alternative to the Advanced Passenger Train in the early 1970s, the British Railways Board authorised the development of a prototype High Speed Diesel Train, with two locomotives designated as Class 41. These aerodynamic power cars were constructed by British Railways Engineering Ltd at their Crewe Works and emerged in June and August 1972, fitted with Paxman Valenta 12RP200L engines, developing 2,250 hp.
The power cars, having initially been numbered 41001 and 41002, were later given the coaching stock numbers 43000 and 43001 for operating trials on the Eastern Region and subsequent transfer to the Western Region. Following evaluation and a change of name to High Speed Train, British Rail placed orders for similar trains for use on the Western, Eastern, Scottish and London Midland Regions.
When originally built at BREL's Crewe Works, the InterCity 125 units were considered to be diesel multiple units and allocated as Class 253 to the Western Region and Class 254 for the Eastern Region. With the introduction of Trailer Guard Second (TGS) carriages, later power cars had no guard's equipment installed and by 1987 most power cars were simply classified as Driving Motor (DM), although they still had luggage van space, retaining a window by the luggage door on each side.