Pöyry will carry out most of the design work needed for 5.5km of tunnels close to the centre of Oslo. The work includes relocating the underground river Alna. Condotte was awarded the engineering procurement and construction contract for this section from Norway's infrastructure manager Jernbaneverket in February 2015.

The 22km Follo Line, connecting Oslo with Ski, is being constructed mainly in tunnel and will be designed for 250km/h operation. The line is needed to meet increasing demand for rail travel caused by the growing population in the Oslo area, and will serve an estimated 150,000 passengers per day when it opens the end of 2021.