To replace steam-hauled trains on secondary lines, Valmet started to build light four-axle railcars in 1954 which were licensed from Hilding Carlssons Mekaniska Verkstad of Sweden. Each motor car had a 180 hp Valmet diesel engine that drove the inner axle on each bogie. In the first year, 15 motor cars were built which were designated Dm6. Since their wheels were too small, the other 197 motor cars were built with larger wheels and designated Dm7. Additionally, 202 trailers were built.

These vehicles were the largest series of diesel railcars of the VR. Aside from rural lines, they were also used on the suburban lines of Helsinki. Depending on the demand, up to eight motor and trailer cars were combined. The withdrawals of the first ones started in 1974 with the cancellation of passenger traffic on secondary lines and the electrification of other lines. In 1988, all had gone. Today around 20 are being preserved.