The Lambertseter Line () is a line on the Oslo Metro which runs from Brynseng to Bergkrystallen. It further shares track with the Østensjø Line along the section from Tøyen to Brynseng. The line runs through a primary residential area of Nordstrand, serving neighborhoods such as Manglerud, Ryen and Lambertseter. The line is served by Line 4 of the metro, which runs every fifteen minutes. This is supplemented by Line 1 that is extended to Bergkrystallen between 6:30 and 19 on weekdays, giving a combined frequency of eight trains per hour. Originally the Lambertseter Line was proposed as an extension of the now closed Simensbråten Line. Planning of a metro started in 1946 and instead of running via Ekeberg the Lambertseter Line was to run via Etterstad. Construction was tied with large-scale construction of housing along the route. As the Lambertseter Line could be completed much earlier than the metro's Common Tunnel, the Lambertseter Line was first built as part of the Oslo Tramway. Plans were approved in 1954 and the line opened on 28 April 1957. Until 17 May 1966 it was operated by Oslo Sporveier's SM53 trams and connected to the city center via the Vålerenga Line. From 22 May it became the first part of the Oslo Metro. The metro's main depot, Ryen Depot, is situated along the line. The Lambertseter Line received a major overhaul between 2010 and 2013. Route The Lambertseter Line runs from Tøyen on the Common Tunnel to Bergkrystallen, a distance of . The segment from Tøyen to Brynseng can arguably be regarded as part of the Common Line or as part of the Lambertseter 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