The British Railways diesel mechanical multiple units (DMU) were largely a development of the original GWR railcars. The units which were constructed by the railway workshops at Derby and Swindon as well as a number of external suppliers such as Metropolitan Cammell and Cravens. The units featured much commonality and common components which simplified their maintenance and operation. The majority of the fleets were powered by 2 British Universal Traction (BUT) 150 bhp engines per power car. Standard formations included single cars, power car and trailer, twin power cars, two power cars plus one (or two) intermediate trailers. A number of higher power units were also developed with 180 bhp and 230 bhp engines. These engines were relatively simple and robust and had been extensively used to power buses. The majority of the units featured a 4 speed epicyclic gearbox which was located after a fluid coupling and a free-wheeling coupling. The gearbox was air operated and all gears were constantly in mesh and gears being selected by means of a friction brake band. The top (4th) gear was operated via an external clutch which locked the whole gearbox resulting in the inlet and outlet rpm being the same (1:1 ratio). The fluid coupling was provided to eliminate the need for a friction clutch thereby allowing a smooth take-up from rest while the free-wheeling coupling was provided to prevent the engine being over-revved when running downhill faster than the maximum engine speed.
The engine was a either a BUT 150bhp (AC220AC) of 11.3 litres capacity or a Leyland 680 of 11.1 litres capacity and the same 150 bhp output. The idling speed was 375-400 rpm, later increased to 410-430 rpm and maximum power was delivered at 1 800rpm. Above 1 800 rpm the power rapidly dropped off with fuel cut-off occurring between 1975 rpm and 2 000 rpm.