million krone. It was taken into use on 24 November 1997, but not officially opened until 4 January 1998. From 12 April 2003 the Østensjø Line was linked to the Sognsvann Line in such a manner Line 4 ran all the way to Sognsvann. This lasted until 9 December 2012, when it was shifted so that it instead ran along the Ring Line and terminated at Storo. In the meantime the T1000 trains were replaced with MX3000 trains. The line was closed in April 2015 for extensive maintenance and upgrades. Upon its reopening in April 2016, the weekday daytime frequency increased from four to eight trains per hour. Four trains per hour run through the city center and connect to the Kolsås Line, while the four other trains only run from Mortensrud to Stortinget and are cancelled during the school summer vacation. The evening and weekend service is four trains per hour, with all trains running the full Mortensrud-Kolsås route. Future Ruter has looked into the possibilities of extending a rail-based transit further south in Nordstrand, to serve the existing residential areas of Loftrud and Bjørndal, and the proposed housing areas of Gjersrud and Stensrud. Ruter have suggested either extending the Østensjø Line or the tramway's Ekeberg Line. The metro proposal calls for six new stations, one at Lofthus, south of Mortensrud, one at Bjørnholt, from Mortensrud, two serving Bjørndal (), one at Gjersrud () and one serving Stensrud (). Travel time from Mortensrud to Stensrud would be 8 minutes and the proposed terminus would be 32 minutes and from Stortinget. Overall investments would be about 2.1 billion kroner. The extension would receive an estimated 14,000 boarding passengers on an average weekday. Handling of this much increased traffic will minimum require eight trains per hour along the Østensjø Line. References Bibliography Oslo Metro lines Oslo Tramway lines Railway lines opened in 1923 1923 establishments in Norway