Two men were arrested early on Saturday evening at Huntingdon Station, around 120km north of London, over what Prime Minister Keir Starmer described as an "appalling incident".
One eyewitness told Sky News that one of the suspects, waving a large knife, was tasered by police.
British Transport Police said the stabbings have been declared a "major incident".
"Ten people have been taken to hospital with nine believed to have suffered life-threatening injuries," a statement on Sunday said.
"This has been declared a major incident and Counter Terrorism Policing are supporting our investigation whilst we work to establish the full circumstances and motivation for this incident."
"We're conducting urgent enquiries to establish what has happened, and it could take some time before we are in a position to confirm anything further," British Transport Police Chief Superintendent Chris Casey said.
"At this early stage it would not be appropriate to speculate on the causes of the incident."
The attack took place as the train headed south towards Huntingdon, a market town a few kilometres northwest of the university city of Cambridge.
Sir Keir said his "thoughts are with all those affected" after the "appalling incident".
Emergency services, including armed police and air ambulances, responded quickly as the train drew into Huntingdon. The attack appears to have been contained swiftly after the train arrived at the station.
British Transport Police, said "multiple people" were stabbed on the Doncaster to London King's Cross train as it headed into Huntingdon. It did not provide a motive for the attack.
Cambridgeshire Constabulary, the local police force, said armed police attended the incident after officers were called to the scene at Huntingdon station at 7:39pm on Saturday (6:39am AEDT on Sunday).
Witnesses said they saw a man with a large knife and passengers hiding in the toilets to escape the rampage, The Times reported.
There was "blood everywhere" and people were getting "stamped" on by others as they tried to flee the horror, a witness told the paper.
"I heard some people shouting we love (you)."
The Sun said another witness said the attack "sparked pure panic on board".
"There was blood everywhere, it was a terrible scene, really violent. It was like something from a film," the witness told the paper.
Video footage on social media showed scores of blue-light police cars and emergency vehicles in a station car park, and a team of armed police running towards the stationary train at Huntingdon station.
Paul Bristow, the mayor of Cambridgeshire and Peterborough, said he had heard of "horrendous scenes" on the train.
London North Eastern Railway, or LNER, which operates the East Coast Mainline services in the UK, confirmed the incident had happened on one of its trains and urged passengers not to travel because of "major disruption".
with PA and Reuters
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