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.

When the number of passengers increased steadily after the Second World War, the NZR tried to gain as many passengers as possible from the developing road traffic. To do this, the steam trains had to be replaced with more modern types of transportation and, above all, the mixed passenger and freight trains had to be replaced with a more modern form. The tender for a modern 88-seater railcar was won by the English Drewry Car Company, with Fiat supplying the engines. The vehicles, which were 32 meters long in total, each consisted of two car bodies that shared a common Jakobs bogie in the middle. In each half was a motor that delivered its power electrically to two axles.

Shortly after the delivery of the 35 vehicles, however, the first problems became apparent. When used on the mountainous routes on the North Island, overheating or problems with the electrical system were common. As a result, the vehicles often had to run with just one engine and were late. In addition, glowing soot particles caused embankment fires during operation under full load. These problems were not resolved until Fiat technicians traveled to New Zealand to investigate. This ultimately led to the engines being derated from 210 to 185 hp each and a possible second order of 15 units did not materialise.

Despite the derating, cracks in the crankshafts continued to occur. Thus, in 1967, the procurement of replacement engines for all vehicles was put out to tender. Due to government regulations, however, only new crankshafts could be ordered, which limited the remaining service life of the vehicles. The service of the railcars was soon discontinued on some connections and replaced by locomotive-hauled trains. From 1976 some examples were converted into unpowered passenger cars, repainted green and called “Grassgrubs”. In the same year, major maintenance work on the remaining railcars was suspended, so that they were all retired by 1978.