August 1994, after a tender had been won by Scandinavian Rock Group (SRG), a collaboration between Målselv Anlegg, Nor Entreprenør and Peab. The original contract price was NOK 541 million. An average of 388 man-years were expended during construction, and the work involved the removal of 1.62 million tonnes of rock. There were three excavation points (headings), located at Bryn, Starveien on the Oslo–Lørenskog border, and at Stalsberg, outside Lillestrøm. The tunnel is long with a cross-section, making it the longest tunnel in Norway when it opened, and the second longest since the opening of the Lærdal road tunnel. It allows speeds of , with a slope of 0.2–0.4% slanting upwards towards Lillestrøm. This provides a natural updraft in the direction of Lillestrøm, but a horizontal ventilation system was needed to work in the opposite direction. This is supplemented with vertical ventilation shafts at Bryn and Starveien. The tunnel is between and below ground, being at its deepest in the vicinity of Bryn. Trains using the tunnel must be capable of a minimum speed of . The tunnel has overhead wires with 15 kV AC railway electrification (). Leakages The tunnel needed to pass under Østmarka, a recreational area east of Oslo. The geology of this area is unstable, and was not ideal for tunnel boring. Pressure for its completion to coincide with the opening of the new airport resulted in tunnel excavation proceeding at maximum speed, without adequate measures to exclude water, which resulted in leaks. Houses above the route of the tunnel in Hellerud were damaged and the water levels of many lakes in Østmarka were reduced, including lakes Lutvann and Nordre Puttjern. After the damage was discovered on 3 February 1997, legal actions were initiated by the Norwegian Water Resources and Energy Directorate requiring remedial measures to halt the leakage. At its worst, the rate of leakage into the tunnel was of water per minute. The Rhoca-Gil proprietary sealant process was used to stop the leaks. On investigation, Rhoca-Gil was revealed to contain a toxic substance called acrylamide, which caused health problems for the tunnel workers. The removal of Rhoca-Gil and its replacement with concrete resulted in additional delays. In addition, an extensive permanent pumping system was installed to restore and maintain lake levels in Østmarka. Curing the leaks and cleaning up the contamination delayed the tunnel's completion by a year, and it was finally opened on