A/S Holmenkolbanen was a company that owned and operated part of the Oslo Tramway and Oslo Metro in Norway from 1898 until 1975 when services were taken over by the majority owner Oslo Sporveier. Holmenkolbanen opened the Holmenkoll Line in 1898, and expanded it to become the first Nordic underground railway in 1928. The company took over operations of the Smestad Line in 1933, the Sognsvann Line in 1934. The company was merged into Oslo Sporveier in 1992.

The Common Tunnel, sometimes called the Common Line, is a 7.3-kilometer (4.5 mi) long tunnel of the Oslo Metro which runs through the city center of Oslo, Norway. The name derives from the fact that all five lines of the metro use the tunnel, which runs from Majorstuen to Tøyen. The section has six stations, including the four busiest on the metro.