construction of tramways were halted. Planning of the metro commenced in 1946. A committee was appointed, who quickly that a tramway would be insufficient to handle the necessary traffic. An overground system was ruled out because of the increased estimates in traffic and an underground route was instead pursued. This part of the line would need to handle a traffic of 20,000 passengers per hour. Following the 1948 merger of the municipalities of Oslo and Aker, it was decided that the Nordstrand area in the former Aker would be opened to large-scale residential construction. Preliminary work on the line planning concluded in 1949 and instead a permanent municipal agency was established, the Planning Office For the Suburban and Underground Lines, on 15 September 1949. Instrumental in the change of magnitude was the change of was a shift in the zoning planning. There 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 Grorud Line; the existing Østensjø Line would be converted to metro. The Lambertseter Line was politically approved on 3 April 1952 and the full four-line network was approved on 9 December 1954. The latter included moving the terminus to Jernbanetorget. Investments were estimated at 221 million Norwegian krone, including rolling stock. The Lambertseter Line was given a new route west of Ryen, and would connect to the Østensjø Line at Bryn. These plans were passed by the city council on 9 December 1954, and also included the construction of the Grorud Line and an extension of the Østensjø Line from Oppsal to Bøler. In February 1956, it was also decided that the Lambertseter Line would be expanded to Bergkrystallen. Construction Because of the long construction time for the Common Tunnel, the plans involved that the Lambertseter Line would first be built as a tramway and later connected to