By 1993, the western end had been upgraded to metro standard, Valkyrie plass was closed, and the first trains ran through the whole tunnel.

The Common Tunnel is a 7.3-kilometer (4.5 mi) long double track rapid transit line running east from Majorstuen to Tøyen and onwards to Brynseng.

At Tøyen, the Grorud Line branches and continues past Carl Berners plass as part of the same tunnel.

[1] From Carl Berners plass to Ensjø, there is a single-track branch, which allows trains access to the other eastern lines without having to change direction.

On 9 June 1911, the city council had nonetheless changed its mind, and voted to accept the tunnel if the terminus was moved to the intersection of Karl Johans gate and Ruseløkkveien, but this was rejected by the company.

[10] Thirty properties sustained cracks and other damage, and the owners threatened Holmenkolbanen with a lawsuit to reclaim the cost of repairs.

[10] By October 1914, the tunnel works had been completed from Rosenborggaten to the statue of Karl Johan in the Palace Park (Slottsparken).

The executive committee supported terminating the tunnel at Eidsvolls plass, but this was rejected by the city council on 13 July 1920.

In September 1921, the committee proposed creating a temporary terminus, following Holmenkolbanen's route, in the square behind the National Theatre (Norwegian: Nationaltheatret).

[13] In June 1923, the executive committee voted to allow a temporary terminus at Nationaltheatret, given that Holmenkolbanen would be obliged to continue the line onwards to Studenterlunden.

[14] A change of plans moved the line's route to Valkyriegaten under Valkyrie plass, which allowed the cavity from 1912 to become a station,[16] despite it only being 300 meters (980 ft) from Majorstuen.

Past Slottsparken, the original track had been built at an increased depth to allow for a future extension of the Drammen Line of the Norwegian State Railways (NSB) to run above the light rail tunnel.

The Sognsvann Line was to be connected to the tunnel from 10 October 1934, and the two companies needed to find an agreement for paying for trackage rights.

[19] After negotiations failed, a decision was made by the Ministry of Labour on 7 July 1932, which stated that Akersbanerne would have to pay the additional fares collected for transport passengers through the tunnel, less the cost of running trains on the section.

[20] On 16 May 1931, the Supreme Court found Holmenkolbanen guilty in the lawsuit regarding the real estate damage, with the compensation payments making the company insolvent.

[24] The first specific plans were launched in March 1954, consisting of four branches which would connect to a common underground line from Tøyen to the city center.

The original plans called for the use of 600 to 650 volt (V) direct current (DC) fed via a pantograph, to allow comparability with the light rail.

The system also used cab signaling and moving blocks, which were cutting-edge technology at the time, and had previously only been implemented on the Stockholm metro in Europe.

[29] Simultaneously, Oslo Sporveier worked on possibilities to connect their eastern and western networks, and extend both to the city core.

The initial plans called for a station at Slottsparken, close to Nationaltheatret, which would serve as the transfer point between the metro and the western light rail.

[31] The initial plans were met with criticism, following the media's discovery that the main planners had not consulted several hired specialists, and that alternatives to the preferred route had not been considered.

[30][32] By 1975, the plans were changed so that Nationaltheatret would become the transfer station, by building a balloon loop for the metro, while allowing the western trains to terminate as before.

[35] In 1978, the city planner discarded the proposal by Oslo Sporveier to build a new station at Slottsparken, and instead decided that Sentrum would become the interchange between the two systems.

[43] Oslo Package 3 is a political agreement between local and national politicians whereby state and municipal grants are combined with revenue from the toll ring to finance NOK 58 billion worth of transport infrastructure investments between 2008 and 2027.

The current station causes long transfer distances between the metro, tram and bus, in part because passengers have to cross Kirkeveien at street level.

The proposed station would have an island platform, easing transfer between westbound lines, and allow access to both sides of Kirkeveien.

[46] NOK 700 million has been allocated to build a new station at Homansbyen, in the residential area of Frogner, roughly midway between Nationaltheatret and Majorstuen.

Because the station would be deep underground, travel time gains to the city center for local residents would be small, compared to using the tram.

[51] Plans from 2011 have called for a new tunnel which runs from Majorstuen via Stortinget to Tøyen, but following a different route and with intermediate stations at Bislett, Hammersborg and Olaf Ryes plass, in the St. Hanshaugen and Grünerløkka boroughs north of the city center.

With the new tunnel being made a high priority by the city planners, this will either require additional grants from the municipality or the state, an increase in the toll fees, or the delay of investments in the program to a future Oslo Package 4.