British police and ambulance services said a number of people had been stabbed on the train, which stopped at Huntingdon, with armed officers seen entering the train in videos on social media.
Cambridgeshire police said they were called at 1939 GMT after reports that multiple people had been stabbed on a train.
"Armed officers attended and the train was stopped at Huntingdon, where two men were arrested. A number of people have been taken to hospital," the police said in a statement.
The East of England Ambulance Service said it mobilised a large-scale response to Huntingdon Railway Station, which included numerous ambulances and critical care teams, including three air ambulances.
"We can confirm we have transported multiple patients to hospital," it said.
One eyewitness told Sky News that one of the suspects, waving a large knife, was tasered by police.
Starmer posted on X that the incident was "deeply concerning."
"My thoughts are with all those affected, and my thanks go to the emergency services for their response," he said. (Reporting by James Davey; Editing by Kate Holton, Franklin Paul, Rod Nickel)