The incident took place on late Friday in a train of the Odakyu Electric Railway company, the NHK broadcaster reported.
A female student sustained seven stabbing injuries in her back and is in a critical condition, while other victims are in non-threatening condition.
Some 1.5 hours after the attack, a man came to a shop and asked the owner to call the police, admitting his guilt.
The 36-year-old unemployed man explained the attack on the young woman with a wish to murder her that emerged when he saw her happy face. The suspect added that he had chosen a train for the attack to maximize the number of victims, who could not flee in the closed space.