All five people on board were injured, none of them seriously, when the train derailed on a stretch of track between Nykirke and Holmstrand, south of Oslo. Having come off the rails, the train smashed into a telephone mast and a bank of rock.
The train was travelling at a speed of 135 kilometres per hour when the brakes were applied, AIBN said on Friday.
Trains are restricted to a speed limit of 130 kilometres per hour on a relatively new stretch of track just before the spot where the accident took place, but this drops to 70 km/h at a point where trains switch to older rails on a curve.
The train derailed 50-60 metres after the switchover. Operators are warned of the need to slow down 1,200 metres before the change to older tracks.
The AIBN’s investigation showed that the train travelled 340 metres in the 11 seconds it took from the time the brakes were applied until the train came to a standstill.
“We don’t know why the speed was so high. It could have been an infrastructural problem, a technical error, or human error,” Jeanette Fagerli-Quaino, spokeswoman for rail operator NSB, told news agency NTB.
The stretch of track where the accident took place is in an area with partially automatic train control. Had the train been in an area with fully automatic control the train would have braked automatically, Fagerli-Quaino said.
Police said they were aware of the AIBN findings and were also continuing with their own independent investigation.
The train was part of NSB's new Flirt (Fast Light Innovative Regional Train) fleet, a model made by Swiss firm Stadler. The new trains had been scheduled to start running on regional and local networks across Norway from February 29th but the start date was postponed indefinitely after Wednesday’s accident. .
NSB has ordered 50 Flirt trains from Stadler at a cost of 4.2 billion kroner ($730 million). The Norwegian operator also has an option to buy a further 100 trains.
Flirt trains can reach a maximum speed of 200 kilometres per hour.