The first underground railway to serve Oslo was the extension of the
Holmenkoll Line to
Nationaltheatret in 1928. The company,
Holmenkolbanen, had plans for further extensions into the heart of the city, but high costs during the construction of the initial tunnel hampered progress. In 1954, the
Parliament of Norway decided to build a rapid transit system for Oslo. Four lines to the east of the city would be built, and in 1966 the section from
Tøyen to
Jernbanetorget opened by
Oslo Sporveier. Jernbanetorget was located adjacent to
Oslo East Station, the main railway station serving Oslo. However, the eastern lines also did not extend into the core of the central business district of the capital. During the 1960s, the
Oslo Tunnel was being planned to connect the
Drammen Line to Oslo East Station. This would make the
West Station unnecessary, and would allow a central station to be built at the location of the East Station. The tunnel was planned to run in a similar route as would be necessary to connect Jernbanetorget to Nationaltheatret, and a railway station was planned to connect to Nationaltheatret. Simultaneously, Oslo Sporveier worked with possibilities to connect their eastern and western networks, and get both into the city core. During the 1960s, the planning office for the metro proposed to build a connecting tunnel between Nationaltheatret and Jernbanetorget. However, the initial plans involved creating a station close to Nationaltheatret, named for
Slottsparken, which would serve as the transfer point between the two systems. This station had a planned dimension to hold 25,000 people. At the time, the western and eastern networks were incompatible; in addition to different platform heights, the eastern network used six-car trains with a
third-rail power supply, while the western network used
overhead wires and two-car trains. The initial plans were met with criticism, following media's discovery that the main planners had not consulted several hired specialists, and that alternatives to the preferred route had not been made. As a consequence, several engineers working for the
Norwegian State Railways made two alternative suggestion for the route. By 1975, the plans were changed so Nationaltheatret would become the transfer station, by building a
balloon loop for the eastern trains, while the western trains would terminate as before. This solution would allow the two networks to be connected later. In 1978, the city planner discarded the proposal from Oslo Sporveier to build a new station at Slottsparken, and instead decided that Stortinget would become the interchange between the two systems. This would allow the western network to later be upgraded to
metro standard and allow through trains. The proposal was supported by all political parties except the
Labour Party. The decision to build Sentrum Station was taken by the city council on 22 May 1969; the decision also involved that the transfer station be placed at Slottsparken/Nationaltheatret. Construction started in 1972, and the first leaks were revealed. By the time the station opened on 9 January 1977, the leaks had not been removed. During trials in January, it turned out that the tracks were too low for all types of trains to be able to open their doors, causing the tracks to have to be raised. By 1978, the contractor and the municipality felt that the leaks were under control, and the municipality took over the station. In February 1983, the station was closed to finally remove all leaks. At the time, it was expected that the station could reopen in 1984. In 1986, the municipality sued Selmer for the it cost to repair the station. The station reopened on 7 March 1987, with the new name Stortinget. The name derives from the Parliament of Norway () that is adjacent to the station. The Common Tunnel was completed, and the trains from the western network terminated where at the old platforms, where the eastern trains had previously terminated. The eastern trains instead operated through a balloon loop. The station featured a step-free walk between the two systems. With the opening of Stortinget, the metro network was declared finished, after the last extension on the
Furuset Line to
Ellingsrudåsen had been made in 1981. After the opening of Stortinget, only seven new station have opened:
Mortensrud (1998),
Forskningsparken (1999),
Nydalen,
Storo (both 2003),
Sinsen,
Husebybakken (both 2006) and
Løren (2016). In 1993, the
Sognsvann Line and the Common Tunnel were rebuilt to metro standard, and the first through trains started operating on both sides of Stortinget. By 1995, also the
Røa Line was upgraded, and all lines started operating through Stortinget. ==Facilities==