In the southern part of the North Island of New Zealand lies the Wairarapa route, which was originally 115 miles long and with the Rimutaka Incline had a three-mile steep ramp with a gradient of 6.7 percent. Instead of a normal rack system, the Fell system was used, which combines a central rail with horizontal friction wheels. The traffic was handled with class H locomotives and required a locomotive change at the beginning and end of the ramp. In order to speed up traffic, a railcar was developed that was to take over passenger traffic on the ramp at higher speeds in adhesion mode.
The railcars had an unpowered bogie and a single powered axle. The chassis was designed in such a way that there are no collisions with the Fell rail, which is not used here. There was only one driver's cab at one end, so the railcars had to be turned at the end of the line. The drivetrain consisted of a petrol engine from Leyland with ten liters and six cylinders.
The series consisted of seven railcars named after Maori canoes. The six vehicles with the numbers RM 4 to RM 9 each held 49 passengers and the RM 10 had 20 seats and a luggage compartment for three tons of payload. They took over the passenger service on the entire Wairarapa line and not just the Rimutaka Incline. In order to save on fuel costs, all vehicles were converted to diesel engines with the same number of cylinders and displacement from 1940 on. Thereby the power was reduced from 130 to 120 hp.
In 1955, the Rimutaka Tunnel opened, eliminating the need for the ramp of the same name and shortening the route. From now on locomotive-hauled trains could take over the traffic on the entire route and the railcars were retired. In 1969, all four of the seven examples were bought by the New Zealand Railway & Locomotive Society. There, RM 5 was first made operational again using parts from the others. In the period between 1992 and 2017 it was even prepared again for use on the main line.