Laying of tracks started in 1936, and was assisted by two steam locomotives. The work started at Myrdal and reached Reinunga the first year, Kjosfossen Tunnel the second year and the Blomheller Tunnel in 1939. The first train to operate on a regular schedule was in October 1939, when a freight train between Myrdal and Berekvam ran three times a week. However, this service was stopped by the end of the month. With the German occupation of Norway in 1940, work started later in the season, but the German authorities wanted to speed up the work to make the line usable for steam trains that same year. They scheduled the line for completion in 1942. Track laying work was accelerated by increasing the number of workers from 58 to 195. Regular freight operation on the Flåm Line started on 1 August 1940, albeit limited to an axle load of . At the time there were four trains each day, two in each direction.