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. Bærumsbanen had a fleet of Class A trams, and these were along with Akersbanerne's units put into use on the combined service. The first Class B trams were delivered to Oslo Sporveier in 1937, and these put into service on the Bærum–Østensjø Line after the serial delivery started in 1939. They aluminum trams were faster and had better comfort, allowing a single tram could replace a Class A tram and trailer. The Kolsås Line was connected to the Common Tunnel from 15 June 1942 and became part of the underground service. The branch from Jar to the city center was renamed the Lillaker Line and tram service cut short to Jar. The Østensjø Line service thereafter became marketed as the Lilleaker–Østensjø Line. However, a limited number of services continued to Kolsås, Avløs or Haslum. From 1944, the operative responsibility for Bærumbanen was taken over by Oslo Sporveier. The Lilleaker–Østensjø Line was then given the service number 9. The municipalities of Oslo and Aker merged in 1948, and the ownership of the Østensjø Line was transferred to Bærumsbanen. The remaining Class A trams were retired between 1946 and 1958 and replaced with Class E, very similar to Class B, but had been built as street trams in 1939. Class B trams were rebuilt from 1955 so they could operate in trains. Conversion to metro Planning of the metro commenced in 1946. An overground system was ruled out because of the increased estimates in traffic and an underground route was instead pursued. This part of the line would need to handle a traffic of 20,000 passengers per hour. Preliminary work on the line planning concluded in 1949 and instead a permanent municipal agency was established, the Planning Office For the Suburban and Underground Lines, on 15 September 1949. Instrumental in the change of magnitude was the change of was a shift in the zoning planning. There