The Østensjø Line () is a line on the Oslo Metro which runs from Brynseng to Mortensrud. It further shares track with the Lambertseter Line along the section from Tøyen to Brynseng. The line runs through the primary residential areas of Bøler, Østensjø and Søndre Nordstrand. The line is served by Line 3 of the metro. The first section of the line was built by Akersbanerne as part of the Oslo Tramway and connected to the Vålerenga Line at Etterstad. The first section, to Bryn, opened on 18 December 1923, and was extended to Oppsal in January 1926. Services were variously provided by Kristiania Sporveisselskab, Bærumsbanen and Oslo Sporveier. Proposals for making the line part of the metro arose in the late 1940s and the system was approved in 1956. This materialised in an extension of the line to Bøler on 20 July 1958. The Østensjø Line became the third line of the metro on 26 November 1967, the same day the line was extended to Skullerud. The last extension, to Mortensrud, commenced services on 24 November 1997. There are proposals to extend the line further south to Bjørndal and Gjersrud. Route The Østensjø Line runs from Tøyen on the Common Tunnel to Mortensrud, a distance of . The segment from Tøyen to Brynseng can arguably be regarded as part of the Common Line or as part of the Østensjø Line. From Tøyen, where the Grorud Line branches off from the Common Tunnel, the line makes a 180-degree loop under the Tøyen Park. At this point there is also a direct, single-track tunnel which allows non-revenue trains access from the Grorud Line. The line passes under the Gjøvik Line and leaves the tunnel just before Ensjø Station, which is located on a short section of ground-level track. After Ensjø the line enters a new tunnel at Malerhaugen, which includes the underground Helsfyr Station. Immediately afterwards it ascends to ground level and continues to Brynseng Station. In this intermediate section the line passes a maintenance of way depot and splits, with the Lambertseter Line receiving separate tracks at Brynseng. Both Helsfyr and Brynseng are the site of several large offices complexes. At Brynseng lines continue concurrently across a bridge over the Trunk Line and then the Lambertseter Line branch off towards Høyenhall Station. The Østensjø Line continues to Hellerud Station, after which the Furuset Line branches off. The line then turns