The most challenging part of the construction was the 14,580 metres (47,830 ft) railway tunnel from Etterstad, just east of Oslo S, to Lillestrøm.
Romeriksporten is the longest railway tunnel in Norway, and is underneath the recreational area Østmarka, in geologically highly unstable ground.
During the construction in 1997, the water level in some of the lakes above the tunnel, including Lutvann and Nordre Puttjern, fell dramatically.
After they were discovered on 3 February 1997 sanctions were initiated by the Norwegian Water Resources and Energy Directorate requiring leakage prevention measures in the tunnel.
At the worst, 3,000 liters of water per minute leaked into the tunnel.

The substance Rhoca-Gil was used to fix the leakages, but the substance failed to work properly.
Not only did it fail to polymerize and stop the leaks, it also contaminated the surroundings with acrylamide.
The entire process of fixing the leak and cleaning up the toxicity delayed the process of building the tunnel by one year; and it was first opened on 22 August 1999.
Further complications arose due to conflicts between NSB Gardermobanen and the construction company.
Retrospective surveys showed a lack of control and reporting procedures during incidents that should have been addressed in 1995, and were never taken seriously.
About sixty houses received damage due to the construction of the tunnel.
An evaluation performed by the Ministry of Transport and Communications showed that NOK 500 million was used on fixing the leaks; however, the report claimed this was, to a large extent, a waste of money due to inefficient engineering procedures.
The same report criticized the planning and organization of the entire construction of the railway.
The most challenging part of the construction was the 14,580 metres (47,830 ft) railway tunnel from Etterstad, just east of Oslo S, to Lillestrøm. Romeriksporten is the longest railway tunnel in Norway, and is underneath the recreational area Østmarka, in geologically highly unstable ground. During the construction in 1997, the water level in some of the lakes above the tunnel, including Lutvann and Nordre Puttjern, fell dramatically. After they were discovered on 3 February 1997 sanctions were initiated by the Norwegian Water Resources and Energy Directorate requiring leakage prevention measures in the tunnel. At the worst, 3,000 liters of water per minute leaked into the tunnel.
The substance Rhoca-Gil was used to fix the leakages, but the substance failed to work properly. Not only did it fail to polymerize and stop the leaks, it also contaminated the surroundings with acrylamide. The entire process of fixing the leak and cleaning up the toxicity delayed the process of building the tunnel by one year; and it was first opened on 22 August 1999. Further complications arose due to conflicts between NSB Gardermobanen and the construction company. Retrospective surveys showed a lack of control and reporting procedures during incidents that should have been addressed in 1995, and were never taken seriously. About sixty houses received damage due to the construction of the tunnel. An evaluation performed by the Ministry of Transport and Communications showed that NOK 500 million was used on fixing the leaks; however, the report claimed this was, to a large extent, a waste of money due to inefficient engineering procedures. The same report criticized the planning and organization of the entire construction of the railway.
Gardermoen Line