Introduced a few weeks before the Wairarapa railcars in 1936, the Midland railcars were the first railcars in New Zealand which were relatively successful. The main reason for their introduction was to replace inefficient and slow mixed trains on the Midland Line on the South Island. They were basically constructed on the chassis of a Leyland Tiger bus and were the first rail vehicles in the country to be powered by diesel engines. RM 20, the first one had a mechanical gearbox, while RM 21 was built with a torque converter. Also RM 20 later got the same torque converter. 

Initial tests on steep inclines showed that the eight-ton railcars had more than enough power with their 98 hp Leyland 8.6 litre diesel engine. As the Christchurch Press had been in search for a means to convey their morning newspaper westwards, they subsidized the running costs. So each early morning, one of the vehicles started from Christchurch with the current edition of the newspaper. As early as 1941, the two Midland railcars were replaced by larger Vulcan railcars. So both were scrapped in 1942.