The line was built as double track from Oslo to Venjar when it was opened in 1998, but the remaining 4km section was built as single track due to the need for additional bridges and a complicated and expensive 380m-long tunnel through loose soil.

However, the single line has since become a bottleneck for operations and Norwegian infrastructure manager Bane Nor says the additional track is needed to improve inter-city operations.

Work on the NKr 1.6bn ($US 186.2m) project will take place while services continue to operate on the adjacent existing line, with the new track expected to open in autumn 2022.

Construction of the next stage to connect with the existing double-track section at Langset, which was opened in 2015, is due to begin next year.

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Work will be undertaken while operations on the existing line continue.