Bærumsbanen, that had become partially owned by Oslo Sporveier, had a similar suburban tramway to the east of the city center, the Lilleaker and Kolsås Lines, where trams continued along street trams lines until they reached the city center. Also they had a lack of sufficient balloon loops. They turned their trams at Athenæum, and therefore did not serve the important Oslo East Station. An agreement was therefore reached with Bærumsbanen and Oslo Sporveier, where Bærumsbanen would operate a through service from Kolsås to Oppsal, without the need to balloon loops in the city center. The service started on 4 January 1937 and was branded as the Bærum–Østensjø Line.