On 15 September 1949, the Underground Railway Office ("Tunnelbanekontoret") was established to start planning the Oslo Metro, a rapid transit system that would serve the new suburbs in Nordstrand and Groruddalen on the east side of town. The first part of the Common Tunnel had been opened in 1928 by Holmenkolbanen, and the plan was to build a through tunnel connecting the eastern and western suburban lines. The decision to build the T-bane was made by the city council in 1954; the Østensjø Line would be converted to metro standard, and three new lines would be built, with the Lambertseter Line opening as a suburban line on 28 April 1957.