The metro project involved a significant extension of the Østensjø Line. The first part of the extension was built as a tramway; work on the section to Bøler commenced in 1957. This included construction of the first tunnel on the Østensjø Line and resulted in Oppsal Station being moved. The tracks were rearranged at Brynseng where the Lambertseters Line's inbound track was placed under the Østensjø Line's outbound track to avoid a level crossing. Between Brynseng and Høyenhall a four-track bridge was built over the Trunk Line. The first revenue trams ran to Bøler from 20 July 1958. During parts the 1960s, the Østensjø Line was also served by Line 3.