Police were able to apprehend the suspect, who ran from the train when it was stopped due to the incident.
The stabbing was reported to police at 7:49pm on Saturday by several people on board the regional train from near the town of Bålsta, according to news agency TT.
“We received a call to Bålsta Station. A man is believed to have been stabbed in the stomach and he has been taken by ambulance to hospital,” Matilda Isaksson, officer in command at Uppsala Police coordination centre, told TT.
The train was stopped by police request at Bålsta – not a scheduled stop for the service – when a man attempted to escape across the platform but was apprehended shortly afterwards, reports the agency.
He is now held by police under suspicion of attempted murder.
“We are now carrying out investigations of the scene,” Isaksson said.
A number of witnesses to the incident were on board the train and will be interviewed by police, reports TT. One witness told tabloid newspapers Expressen and Aftonbladet that the man became violent after he was found to be travelling without a ticket, turning his anger on a passenger sitting adjacent who had become involved in the situation.
The man who was stabbed sustained serious injuries but is in a stable condition, according to a report from Uppsala University Hospital.
Police have taken the relevant train carriages out of service as evidence and they were moved away from the platform at Bålsta. Sweden's transport administration (Trafikverket) reported no delays on the affected section of track after 10:30pm Saturday.
Staff on board the train were sent home from work and all passengers were taken to their destinations by alternative means, Anders Edgren, press officer with Swedish rail operator SJ, told TT.