By the 1920s it seem evident that the three remaining narrow-gauge mainlines, the Røros, Vestfold and Jæren Lines would be gauge converted. Røros Copper Works shut down production in 1926, citing high transport costs caused by transshipment at Støren as one reason.Bjerke & Stenersen: 135 The next stage of gauge conversion took place in 1931, when the section between Rena and Koppang, which opened on 7 October. By November the dual gauge between Rena and Hamar had been removed.Bjerke & Stenersen: 127 A unison Parliament passed legislation on 3 June 1935 to gauge convert the line. The upgrade was planned to reduce the minimum curve radius to . Four new tunnels were planned to bypass otherwise narrow sections, but in the end only the Drøyli Tunnel was built. The other major works was to replace the branch line which Røros Station was built on with a semi-circular loop through the town. Following the German occupation in April 1940, work on converting the remaining of the line was accelerated.Bjerke & Stenersen: 138 A major flood in Gaula started on 24 August that year, washing away parts of the line several places through Gauldalen and station at Rognes was washed away.Bjerke & Stenersen: 139 The line was improved and placed on a new right-of-way several places.Bjerke & Stenersen: 142 By May 1941 there were 1,723 men working on the line improvements.Bjerke & Stenersen: 144 The conversion itself took place two stations at a time, an dwas completed on 3 August.Bjerke & Stenersen: 145
