 The final extension of the line occurred on 1 January 1930 to Kolsås.From 4 January 1937 Bærumsbanen took over the operation of the Østensjø Line on the east side of Oslo, that was owned by the municipal Akersbanerne—allowing the company to operate through services from Oppsal to Kolsås.To shorten travel time on the outer part of the Lilleaker Line, the company received permission in 1938 build a line from Jar to Sørbyhaugen on the Røa Line, allowing the trains to use the Common Tunnel shared by the other suburban lines in the west to quickly reach the city center at the underground station at Nationaltheatret.