In 1920, Holmenkolbanen applied to build a branch from Frøen via Tåsen to Nydalen and Grefsen, along what is today the Ring Line. This proposal included a branch to Sognsvann. The Ministry of Labour granted the concession to Akersbanerne, who started construction in January 1922. However, due to a labor conflict, the work was put on hold in 1924. In 1924, the Røa Line (at the time called the Smestad Line) was taken over by Akersbanerne, but Holmenkolbanen retained the operation. After the first part of the Common Tunnel, from Majorstuen to Nationaltheatret, opened in 1928, Holmenkolbanen was in financial distress, and in 1932 Akersbanerne took over the majority of the shares of Holmenkolbanen. At the same time, the Røa and Sognsvann Lines were merged into Holmenkolbanen.