Common Tunnel

In 1898, the Holmenkollen Line was opened as a light rail between Majorstuen to Holmenkollen. The terminus at Majorstuen was an inconvenience for passengers as most people were bound for the city center and had to change to the street trams. For the operator Holmenkolbanen, there were two ways to solve the issue. They could sign trackage rights with Kristiania Elektriske Sporvei to use the Briskeby Line to reach the city center. Alternatively, a tunnel could be built to an underground station in the city center.[5] Applications for a tunnel were sent to the municipality in 1901.[5] However, disagreements as to whether the tunnel was to be considered a railway or tramway halted progress; the city considered the line a tramway while the company considered it to be a railway.[6]

A new application was submitted in November 1907, which proposed a single track line to the intersection of Karl Johans gate and Fredriks gate, and was estimated to cost 1.5 million Norwegian krone (NOK). The plans drew criticism from city officials, who stated that the line would take passengers away from the existing tram line and that it would stimulate migration to the areas along the Holmenkollen Line in the neighboring municipality of Aker, thus reducing the tax income for the city.[7] In 1909, Oslo City Council stated in a letter to the government that they were opposed to the tunnel. 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.[7] On 27 May 1909, Aker Municipal Council voted in favor of the tunnel, and on 9 June 1911 the proposal was passed by the Parliament of Norway. Both municipalities wanted the redemption right, which would allow them to purchase the railway at a later date, but this was instead given to the state. On 15 December 1911, Holmenkolbanen received a concession through a royal decree, which would last for 60 years from the date of opening.[8]

Tram at Nationaltheatret in 1928

Construction on the central parts of the tunnel started in 1912. At the time, an agreement had not been reached with the municipality about the location of the terminus.[9] In October, there was a landslip at Valkyrie plass, resulting in a cavity.[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 issue of the terminus was still not resolved, and work was therefore halted. Holmenkolbanen were planning on building the line through the city center, and therefore wanted the terminus at Fredriks gate, while the municipality wanted it at Ruseløkkveien.[9]

In 1915, the city launched a competition for a plan for city's light rail system. However, the invitations were not issued until two years later. The winner was announced in July 1918, and in September 1919, a municipal committee concluded on how the network should be expanded.[11] Firstly, it proposed connecting the Lilleaker Line to Majorstuen. Secondly, it proposed a tunnel through the city center, from Majorstuen to Stortorvet and Vaterland, and onwards as an elevated line eastwards, along routes that closely resemble the current metro lines and the Ekeberg Line. From Stortorvet, a northern branch was proposed which would be built towards Kjelsås. Finally, the committee wanted a circular line north of the city center, closely resembling to the current Ring Line.[12]

The proposal forced the municipality to reconsider the terminus of the tunnel. The executive committee supported terminating the tunnel at Eidsvolls plass, but this was rejected by the city council on 13 July 1920. Holmenkolbanen convinced the municipality to establish a committee to look into the specific issue. In September 1921, the committee proposed creating a temporary terminus, following Holmenkolbanen's route, in the square behind the National Theatre (Norwegian: Nationaltheatret). This was under the condition that experts concluded that construction would be done without damage to surrounding buildings. The proposal was passed by the city council on 13 October 1921. In June 1922, an expert committee concluded that construction would be satisfactory.[12]

Entrance to Nationaltheatret in 1935

Aker Municipality held an option to purchase the Røa Line, should the tunnel not be completed by November 1922. On 11 November 1922, the municipal council executed the option. Aker had recently established the tram company Akersbanerne, and merged the Røa Line into Akersbanerne from 1 November 1924.[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. The company did not accept the new terms, and the city council finally passed a proposal that the company could accept on 3 April 1924. However, by then traffic had increased on the Holmenkollen Line, the Røa Line had been connected to Majorstuen, and the Sognsvann Line was under planning. This caused Holmenkolbanen to change the tunnel to double track. To finance the construction, the company borrowed NOK 11.5 million.[14]

In 1925, Oslo District Court found Holmenkolbanen not guilty in the lawsuit over damages at Valkyrie plass.[15] A concession was granted on 15 January 1926, which demanded that the tunnel open by the end of 1928.[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. By 1926, these plans had been discarded, and the second track was not built as deep.[17]

MX3000 train at Nationaltheatret

The line was officially opened by King Haakon VII on 27 June 1928.[17] It became the first underground railway in the Nordic countries, five years before Stockholm and six years before Copenhagen.[18] The tunnel was 1,620 meters (5,310 ft) long[16] and trams used three to four minutes to run through it.[17] It was used by two companies,: Holmenkolbanen and Akersbanerne, with Holmenkolbanen operating both the Røa Line and the Holmenkollen Line. 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.[21] This resulted in Aker Municipality purchasing the majority of Holmenkolbanen, and merged the Røa Line and Sognsvann Line into the company.[22] In 1939, Bærumsbanen started construction of a branch from the Lilleaker Line to Majorstuen. From 15 June 1942, the Kolsås Line, as it was renamed, started running via the tunnel to the city center. Bærumsbanen became part of Oslo Sporveier in 1971.[23]

Tøyen

In 1948, the Oslo and Aker were merged, and the new city government started planning suburban expansion, particularly in Groruddalen.[2] On 15 September 1949, the Planning Office for the Suburban and Underground Lines was established as a division within the new municipality.[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. Originally the plans called for a terminus at Grønlands torg, with a later extension to Nationalhteatret included. However, it was quickly decided that the terminus instead should be at Jernbanetorget, next to Oslo East Station (today Oslo S).[2]

The Lambertseter Line and the Østensjø Line existed as light rail lines, and were upgraded to metro standard. The Grorud Line and the Furuset Line were new and ran through previously undeveloped neighborhoods in Groruddalen. During construction, groundwork and electrical equipment was contracted to developers, while the trackage was done by the planning office. 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. This was later changed to 750 V DC via a third rail,[25] allowing reduced maintenance costs and a larger contract surface, permitted a larger electric current and more power to the trains. 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. The minimum allowed distance between trains was set to 90 seconds on the common section and 120 seconds on the branch lines.[26] The tunnel, along with the upgraded Lambertseter Line, opened on 23 May 1966.[27] The Grorud Line was connected on 16 October 1966, the Østensjø Line on 29 October 1967, and the Furuset Line on 18 November 1970.[28]

Stortinget

During the 1960s, the Drammen Line was being planned to connect to Oslo East Station through the proposed Oslo Tunnel. This would make Oslo West Station unnecessary, and would allow a central station to be built at the location of the East Station. The railway tunnel was planned with an intermediate railway station at Nationaltheatret.[29] Simultaneously, Oslo Sporveier worked on possibilities to connect their eastern and western networks, and extend both to the city core. During the 1960s, the planning office for the metro proposed a tunnel between Nationaltheatret and Jernbanetorget, which would run parallel to the Oslo Tunnel. 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. This station was designed to handle 25,000 people per day.[30]

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.[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. As a consequence, several engineers working for NSB made two alternative suggestions for the route.[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. This solution would allow the two networks to be connected later.[33]

Stortinget

The decision to build Sentrum Station, located 700 meters (2,300 ft) west of Jernbanetorget,[27] was taken by the city council on 22 May 1969. The decision also involved the transfer station being placed at Slottsparken/Nationaltheatret. Construction of Sentrum started in 1972, and was immediately followed by the first water leaks being discovered. By the time the station opened on 9 January 1977, there were still no adequate countermeasures for the leaks.[34] During trials in January, it was discovered that the tracks were too low to allow all types of trains to open their doors, resulting in the tracks having to be raised.[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. This would allow the western network to be upgraded to metro standard at a later date, and subsequently also allow metro trains to run through the center. The proposal was supported by all political parties except the Labour Party.[36]

By 1978, the contractor and the municipality felt that the leaks were under control, and the municipality took over the station. On 20 February 1983, the station was closed for renovation to remove the leaks. At the time, it was expected that the station could reopen in 1984.[34] The cause of the leaks was a combination of the wrong type of concrete being used and the wrong construction method. These were specified correctly in the tender contracts, but after the contractor was chosen, an agreement was made between the municipality and Selmer for the use of the membrane method. Combined with the inferior quality of concrete, the leaks were unavoidable.[37] In 1986, the municipality sued Selmer for the NOK 158 million it cost to repair the station.[38]

The station reopened on 7 March 1987, and took the name Stortinget, which derives from the Parliament of Norway Building (Norwegian: Stortinget), which is adjacent to the station. Trains from the western network terminated at the old platforms, where the metro had previously terminated. The metro trains instead ran through a balloon loop.[39] The station featured a 40-meter (130 ft) step-free walk between the two systems.[40] 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.[41]

Jernbanetorget in 2008 with a T1300, a class that has since been retired

On 7 October 1987, the city council decided to connect the four light rails west of the city center with the metro, allowing through trains.[40] The Sognsvann Line was selected as the first line to be upgraded. The overhead wires were replaced with a third-rail power supply, automatic train protection was installed, platforms were built long enough to accommodate six-car trains instead of two-car trains, and the platform height was increased.[42] The upgrade also included the installation of third-rail power supply from Majorstuen to Stortinget. From 10 January 1993, the Sognsvann Line re-opened, and from 4 April, the line started operating through the Common Tunnel and connecting to the Lambertseter Line. The new service used T1000 rolling stock.[43]