Construction of the tunnel commenced in 1926. Due to the line running through many sparsely populated areas, many of the guard houses were built early in construction to be used to house workers. By 1933 there were 869 men working on construction, rising to 1,744 three years later. Laying of track started around Brekkvasselv and Mosjøen in 1936. The line was built in NSB's highest standard, with 35 kg/m (70 lb/ft) tracks. The line was planned opened during to Mosjøen in the fall of 1940, and to Elsfjord in November. The German occupation of Norway in April 1940 dramatically changed the pacing of construction. The Nordland Line was given high military priority and the order was given for imminent opening of the line. Three bridges were destroyed during the Norwegian Campaign and new temporary structures had to be put up to replace them or the damages repaired. Operations from Grong to Mosjøen started on 5 July with an official opening two days later. NSB extended the daily Trondheim–Grong train to Mosjøen. In addition military trains were operated as needed. Due to shortcuts in the hasty completion, full operating speeds were not reached until 1 July 1942. With increased use of the line, several Class 63 locomotives were imported from Germany for use on the line. Later Class 30 and 32 were used further south.