 The companies preferred shares were however still listed on the Oslo Stock Exchange.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.