Flytoget (English: The Airport Express Train) is a high-speed airport railway service connecting Oslo Airport to Oslo Central Station. Run by Flytoget AS (formerly NSB Gardermobanen AS), it operates on the high-speed Gardermoen Line using sixteen GMB Class 71 electric trains. Normal service frequency is once every ten minutes, with five of the services each hour continuing westwards beyond Oslo Central. The journey takes nineteen minutes, with extended services serving nine stops within Greater Oslo and taking up to 60 minutes.

Flytoget transported 5.4 million passengers in 2007, a 34-percent market share of airport ground transport. The service, which has a top speed of 210 kilometres per hour (130 mph), is the only high-speed rail service in Norway. Construction started in 1994 and high-speed trains began serving Gardermoen Airport from the date of its opening on 8 October 1998, although full operation using the 14.5-kilometre-long (9.0 mi) Romerike Tunnel had to wait another ten and a half months after severe leaks caused by the tunnel's construction led to the partial depletion of two lakes.

Formed in 1992 as a subsidiary of Norwegian State Railways, the company has been owned since 2001 by the Norwegian Ministry of Trade and Industry.

When the Parliament of Norway on 8 October 1992 decided to build a new central airport for Eastern Norway, they also decided that the main mode of ground transport should be by rail. While the previous airport, Oslo Airport, Fornebu, was located just outside the city limits, the new airport, Oslo Airport, Gardermoen, would be located 50 kilometres (31 mi) north of the city, outside the reach of existing public transport. The principle of the airport construction was that it was not to be footed by the tax payers; the entire airport would be built with borrowed money through Oslo Lufthavn AS, a subsidiary of the Norwegian Airport Administration. The same principle was chosen for the airport rail link—the Norwegian State Railways (NSB) creating the limited company subsidiary NSB Gardermobanen AS, founded on 24 November 1992, to perform the construction of the line. It would be able to charge train operators using the line, channeling the payments to cover down payments and interest of the debt used to build the railway. Profit margin was estimated to 7.5%.

Oslo Airport, Gardermoen, is not on the Trunk Line that runs north from Oslo. With heavy traffic and many small stops until Lillestrøm, and continuing north as single track, the Trunk Line would have to be supplemented by a parallel double track from Oslo, with a new route north of Kløfta to Eidsvoll, the 16 kilometres (10 mi) north of the airport allowing trains operating on the Dovre Line to Lillehammer and Trondheim to access the airport. The 64-kilometre (40 mi) line was named the Gardermo Line.

The Gardermo Line was the second attempt to build high-speed rail in Norway, after the 35-kilometre (22 mi) line from Ski to Moss on the Østfold Line. But no operation speeds exceeding 160 kilometres per hour (99 mph) are achieved there due to short distances and limitations to rolling stock, making Gardermobanen the first real high-speed railway line in Norway. Due to the domination of single track in Norway, the opening of the Gardermo Line increased the total length of double track in the kingdom by two-thirds.

Construction started in 1994. An agreement for purchase of sixteen three-car electric multiple units was signed with Adtranz on 23 February 1995. Parliament decided on 1 October 1996 that the construction company would also operate the new train service. The trains were delivered between 19 September 1997 and 30 January 1998, costing NOK 1.4 billion.

The greatest challenge was the need to build the 14,580-metre (47,830 ft) Romerike railway tunnel – Norway's longest – beneath the geologically highly unstable Østmarka area between Etterstad, close to Oslo Central Station, and Lillestrøm. During construction, in 1997, the water level in some lakes above the tunnel, including Lutvann and Nordre Puttjern, sank dramatically. After the leaks were discovered on 3 February 1997, sanctions were imposed by the Norwegian Water Resources and Energy Directorate requiring the elimination of all leakages. At its worst, the tunnel was leaking 3,000 litres (660 imp gal) of water per minute.