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. Work commenced again in 1933 and the line opened on 10 October 1934. At first the line was built with double track from Frøen to Østhorn and single track to Sognsvann. Between 1937 and 1939, the upper section was converted to double track. The construction spurred a massive residential construction along the line giving a good foundation for ridership. After the municipal merger between Oslo and Aker in 1948, Akersbanerne was merged into Oslo Sporveier, although Holmenkolbanen remained a partially owned subsidiary. In 1973, Oslo Sporveier bought the remaining shares, and two years later the operations were transferred to Oslo Sporveier, with Holmenkolbanen remaining a pure real estate company. The municipal take-over resulted in an upgrade to both the track and the rolling stock. New overhead wires and new and heavier track were laid. From 1978, the T1300 trains were taken into use. With the take-over, the lines were numbered, and the Sognsvann Line designated 81, although this was quickly changed to 13. During the 1980s, the city decided to connect the four suburban lines west of the city center with the Oslo Metro. The Sognsvann Line was selected as the first line to be upgraded, so the two systems would become compatible. The line was upgraded to metro standard by replacing the overhead wire with a third-rail power supply and installing automatic train protection. All the stations were rebuilt, with platforms lengthened from fitting two-car to fitting four-car trains, and the platform height increased. The third rail required all level crossings to be removed, resulting in the line being rebuilt several places to allow the road to underpass or overpass. The light rail permitted passengers to cross the lines at the stations, but this was made impossible with metro standard. Because of this, underpasses or overpasses were built at the stations. Frøen Station was rebuilt so only the trains on the Holmenkollen Line would stop there, while Nordberg Station was closed. All the stations, except Ullevål stadion, were built in the