was a large housing shortage in Oslo and the region was experiencing rapid population growth. The solution was to build a series of commuter towns in Aker, which would be the basis for the traffic on the metro. Plans for both a metro and new housing were substantiated in a 1950 municipal plan. 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. The metro project involved a significant extension of the Østensjø Line. The first part of the extension was built as a tramway; work on the section to Bøler commenced in 1957. This included construction of the first tunnel on the Østensjø Line and resulted in Oppsal Station being moved. The tracks were rearranged at Brynseng where the Lambertseters Line's inbound track was placed under the Østensjø Line's outbound track to avoid a level crossing. Between Brynseng and Høyenhall a four-track bridge was built over the Trunk Line. The first revenue trams ran to Bøler from 20 July 1958. During parts the 1960s, the Østensjø Line was also served by Line 3. The original metro plans had not called for the Østensjø Line to run further than to Bøler. As housing development was gradually forcing its way further south in Nordstrand, the municipal council saw the need for extensions to the line. The section from Bøler to Borgerud was approved on 8 March 1963 and construction started immediately. The section from Borgerud to Skullerud was approved of on 31 January 1964. The total costs of the line from Helsfyr to Skullerud ended at 39 million Norwegian krone. With the conversion to metro standard, the