In the 1900s the GWR built a small fleet of steam railmotors for local stopping services, these often being known by railwaymen as rail'cars' and having a spacious open saloon interior layout in place of the compartments of contemporary mainline coaches. The railmotors were intended to be able to hauling an additional coach of similar design, described as a 'trailer', however the GWR found the compact railmotor engine was rather underpowered when extra coaches were needed. While the last steam railmotor was not withdrawn until 1935, by which time the GWR was testing diesel railcars, the GWR had found that separating the locomotive from the coach allowed a more powerful locomotive to be used. Detaching the coach also kept the furnishings away from the filth of steam locomotive shed.
As the engines wore out the railmotors were converted into trailers, retaining a driving cab end with regulator and brake controls and fitted with 'through control' gear comprising a rod beneath the coach connecting to the locomotive regulator via a flexible coupling. The success of the new small locomotive and control trailer 'auto trains' in local and branch service saw the fleet of dedicated trailers expanded with new cars being built into the early 1950s.