Britains' railways experimented with diesel traction through the early part of the 20th century, trying out a number of single car railbus type units. The GWR put the first successful railcars into service in the 1930s, combining AEC bus diesel engines and streamlined bodywork by Park Royal with railway coach structural engineering to create stylish express railcars. Drive was arranged through a mechanical transmission via cardan shafts and axle-end gearboxes. The GWR assembled a fleet of 38 of these railcars with units designed for branch line, suburban, mainline stopping and express service. The last 4 Swindon-built railcars were arranged as single ended multiple-unit cars,British Railways greatly refined the GWRs multiple unit design in the 1950s. Though the base technology was very similar drive shafts ran under the frames to final drive gearboxes mounted centrally on the axles and more conventional railway passenger coach bodies were fitted. Two principal roles were defined, suburban passenger and cross country. Suburban units had doors to each seating bay to allow rapid loading and unloading. Cross-country units had more spacious saloon seating with door vestibules and were intended for longer distance and branch services.