After the municipal merger between Oslo and Aker in 1948, Akersbanerne was merged into Oslo Sporveier, although Holmenkolbanen remained a partially owned subsidiary. In 1973, Oslo Sporveier bought the remaining shares, and two years later the operations were transferred to Oslo Sporveier, with Holmenkolbanen remaining a pure real estate company. The municipal take-over resulted in an upgrade to both the track and the rolling stock. New overhead wires and new and heavier track were laid. From 1978, the T1300 trains were taken into use. With the take-over, the lines were numbered, and the Sognsvann Line designated 81, although this was quickly changed to 13.