The basics for the metro were established in 1951, with a common segment through Enerhaugen with a terminus at Grønlands torg, to avoid having to cross Akerselva. The plans were made such that in the future the line could be extended to connect to Holmenkolbanen's western underground line at Nationalthatret. Four branches would be built, including the Østensjø Line, would be converted to metro. The conversion and metro were approved by the city council on 9 December 1954. Investments were estimated at 221 million Norwegian krone, including rolling stock, for all four lines. The Østensjø Line would retain its route from Bryn to Oppsal, but it would receive a new section from Bryn to Tøyen, where it would run concurrently with the Lambertseter Line. Construction of the latter started in 1952 and was completed on 28 April 1957, sharing track from Etterstad to Bryn.