scheme was introduced, where full-service trams operated all stations from the city center to Jar, while express trams would only serve boarding passengers from Athenæum to Tjernsrud. The full-services ran every 12 minutes and the express services every 30 minutes. The full-service routes were reduced to a 15-minute headway in 1932, but a 15-minute headway was introduced to Bekkestua with the express service. During rush-hour, there was a 7-minute headway to Jar and additional services to Haslum. One of the conditions for the concession of 1924 was that the line was to be connected to the Common Tunnel of the Holmenkollen Line. Oslo Municipality demanded that the company either turn its trams at Skøyen or allow the municipality to purchase the company cheap. Bærumsbanen was bought by Oslo Sporveier on 1 October 1934 and turned into a subsidiary. The two companies had a common executive management, but retained separate operations. At the same time, Aker Municipality's tram company Akersbanerne was left operating a single line east of the city center, the Østensjø Line. In 1936, it was decided that Bærumsbanen would take over operation of this line, by operating the trams from the Kolsås Line through the city center to connect with the Østensjø Line. Operations started on 4 January 1937, and the service was marketed as the Østensjø–Bærum Line. Although Bærumsbanen took over Akersbanerne's three Class A trams, up to three units from Ekebergbanen were also leased. Underground connection Since the birth of the line, plans had existed to connect to the Common Tunnel of the Holmenkollen Line. The tunnel would allow the trams to operate straight to the city center at Nationaltheatret, giving faster travel times and removing the heavy Class A trams from the city streets. A plan for the route from Jar to Sørbyhaugen on the Røa Line had been made in 1919, but by the 1930s, the Norwegian Radium Hospital and a tuberculosis hospital at Ullern had been built in the immediate vicinity of the line. Concession for this route was granted on 30 April 1937, but Aker Municipality protested, because they did not want noise pollution at the hospitals. Another concern was that the magnetic field from the overhead wires could affect the instruments at the hospitals. The route was therefore moved, but this made construction more difficult due to worse geological conditions. Construction started in 1939, but was put on hold in 1940