For rural lines with a small number of passengers, the GWR started to acquire steam rail motors starting in 1903. They were not only cheaper to operate and maintain, but also allowed to set up additional halts without personnel and without high platforms due to their built-in steps. What made them look more modern than other contemporary steam rail motors was that they didn't have a power unit that resembled a small tank locomotive, but they had a vertical boiler that was mounted on the power bogie and hidden inside the car body.

A total of 99 vehicles of a number of sub types called diagram A to diagram R were built between 1903 and 1908. They had lengths of 57 to 70 feet, a width of either 8.5 or 9 feet and cylinders of different sizes. While the first batches didn't have a baggage compartment and were dubbed “suburban”, the later batches had one and were dubbed “branch”. A total of 112 power bogies had been built for these 99 cars to increase their availability during maintenance.

Although there were also matching trailers with a driving compartment for these rail motors, they were not as flexible in times of varying demand as loco-hauled trains. On lines with steep gradients, they had less power reserves than loco-hauled trains or new petrol railcars. So the last one was withdrawn in 1935 and only No. 93 was preserved. Today it is operational and can often be seen accompanied by trailer No. 92.