Between 1938 and 1942, the Vulcan Foundry in England built ten railcars for the New Zealand Railways. Like all railcars of the NZR, they were put into class RM for “rail motor”. To prevent turning of the vehicles, they had one cab on each end. Between those, there was space for 50 passengers, a luggage compartment and a toilet. 250 hp came from a Frichs diesel Engine built under license by Vulcan. To reduce the axle load on the powered bogie, this one had received an additional carrying axle. 

Although designed to run at 75 mph or 120 km/h, they had a top speed of 55 mph or 88 km/h in service. On October 25, 1940, RM No. 50 reached a speed of 78 mph or 125.5 km/h. This was a rail record for New Zealand and today it is still the highest officially recorded speed. After only running on the South Island, the last one of this type was withdrawn in 1978.