It further shares track with the Østensjø Line along the 2.5-kilometer (1.6 mi) section from Tøyen to Brynseng.

The line runs through a primary residential area of Nordstrand, serving neighborhoods such as Manglerud, Ryen and Lambertseter.

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.

Until 17 May 1966 it was operated by Oslo Sporveier's SM53 trams and connected to the city center via the Vålerenga Line.

The Lambertseter Line runs from Tøyen on the Common Tunnel to Bergkrystallen, a distance of 8.4 kilometers (5.2 mi).

[2] At this point there is also a direct, single-track tunnel which allows non-revenue trains access from the Grorud Line.

[2] After Ensjø the line enters a new tunnel at Malerhaugen, which includes the underground Helsfyr Station.

[9] After the latter the line turns ninety degrees, passes through a 245-meter (804 ft) tunnel before reaching the terminus, Bergkrystallen.

Operations of the lines are done by Sporveien T-banen on contract with Ruter, the public transport authority in Oslo and Akershus.

[14] The first plans to build a tramway along part of the route of the Lambertseter Line were launched by Ekebergbanen.

Ekebergbanen proposed extending the branch along the route of the Lamberseter Line between Ryen and Brattlikollen.

An overground system was ruled out because of the increased estimates in traffic and an underground route was instead pursued.

There was a large housing shortage in Oslo and the region was experiencing rapid population growth.

The solution was to build a series of commuter towns in Aker, which would be the basis for the traffic on the metro.

[18] The basics for the metro were established in 1951, with a common segment through Enerhaugen with a terminus at Grønlands torg, to avoid having to cross Akerselva.

[15] Because of the long construction time for the Common Tunnel, the plans involved that the Lambertseter Line would first be built as a tramway and later connected to the metro.

To ease the conversion, the Lambertseter Line was built for 3.2-meter (10 ft 6 in) wide trains, lean curves and no level crossings.

Between Munkelia and Bergkrystallen, the 245-meter (804 ft) Rabben Tunnel was built, and a branch line to the brick factory at Høyenhall.

However, lack of rolling stock and other technical problems caused Oslo Sporveier to alter the plans on 16 March 1966, and instead opted to open the Lambertseter Line first.

With the upgrade, T1000 rolling stock was taken into use, along with automatic train control, step-free access with high platforms and a third-rail power supply.

Their poor quality had among other issues caused the speed between Høyenhall and Manglerud to be reduced to 30 kilometers per hour (19 mph).

Three stations, Høyenhall, Manglerud and Ryen, received upgrades, including new sheds, new lighting and better accessibility.

[33] A second round of upgrades were carried out from 8 April to 9 September 2013, this time on the section from Ryen to Bergkrystallen.

All stations except Bergkrystallen were upgrades, receiving new sheds, lighting, platform decks and improved accessibility.

Tracks and ballast were replaced, allowing the temporary speed limit of 40 kilometers per hour (25 mph) to be abolished.