One of the victims was taken to hospital with cuts.
On Thursday, the police moved out to Sandvika with a large number of resources, the police report on Twitter.
A gang of between six and 10 people attacked three people on board the train when it was at Sandvika station.
Task leader Arve Røttingen said on the spot.
The people involved are young, probably between 16 and 19 years old, according to the task leader.
A boy has a cut on his arm after being stabbed by a sharp object, and has been taken to hospital. He is not seriously injured.
According to the police, the boy ran from the scene after the incident, before he was found inside Sandvika shopping centre.
Looking for more
A search is underway for several people involved.
The police have not yet clarified the course of events, nor the background to the incident.
– We speak to witnesses, those involved and look through video surveillance. We’ll see if it produces results, says Røttingen.
It is unclear whether there is any relationship between those involved.
– We don’t know if they are friends or if they met on the train, says the task leader.
Witnesses and those involved are being questioned, the police write.
