The Lilleaker Line, today known as Tram 13, was voted by Aker Municipality Council on 8 November 1914. The construction work started the year after, and the line was opened on 9 May 1919. After the line was extended to Bekkestua on 1 July 1924, it was renamed the Bærum Line. On 3 November it was further extended to Haslum, and to Kolsås on 1 January 1930. From 4 January 1937 there were direct tram services between Kolsås and Oppsal on the Østensjø Line.
New corridor
After a new tunnel opened between Majorstuen and Nationaltheatret in 1928, the rail company A/S Bærumsbanen wanted to construct a new corridor between Jar and Sørbyhaugen on the Røa Line. The construction work started in the end of 1938 and was completed when the line opened on 15 June 1942. The new corridor, including the stretch to Kolsås, was now named the Kolsås Line, and the old corridor the Lilleraker–Østensjø Line. From 29 October 1967 the Lilleaker–Østensjø Line was split into two lines: The Østensjø Line and the Lilleaker Line. The first became a metro line from 29 October and the latter was connected with the Ekeberg Line on 29 September 1974. A new corridor opened on the tram line between Thune and Hoff on 17 November 1981.
Temorarily closed
On 16 February 2009 the stretch between Lilleaker and Bekkestua was closed since the Kolsås Line was upgraded to metro standard. The stretch between Lilleaker and Jar reopens on 1 December 2010, and the remaining part to Bekkestua on 19 January 2014. After the reopening the Ringstabekk station was permanently closed.
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