The VR listed the first heavy mainline diesel locomotives, which were built from 1959 to replace steam locomotives, as Hr12. With a six-axle chassis, an output of 1,400 kW and a maximum speed of 120 km/h, heavy goods and passenger trains alike were to be transported.

The engine came from MAN and was manufactured under license by Tampella. It had 16 cylinders with a displacement of 182 liters in total. The axle load was more than 20 tonnes, but this was not a problem for the planned area of work on main routes. Steam heating was available at the time to heat passenger trains. Of the 42 pieces produced, only the last ten were designed for a top speed of 140 km/h, although this was not used in reality and no certification for higher speeds was given.

With the extinction of passenger cars with steam heating, the steam boilers were later removed and the locomotives were only used in front of freight trains. Compared to newer diesel locomotives, it became clear over time that the engine power of the locomotives, now known as Hr12, was relatively low in comparison to the weight. This made them predestined for heavy, slow freight trains in later years. After the large main lines in Finland were electrified, the locomotives could not be used on many of the remaining non-electrified branch lines due to the high axle load. The locomotive, which was the last Dr12 to pull a freight train in December 1990, was preserved for posterity.