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.