Two suspects were in custody and investigators were working to find a motive, British police said on Sunday.
Bloodied passengers spilled out of the long-distance train when it made an emergency stop in the town of Huntingdon, where dozens of police waited, soon after multiple stabbings were reported onboard.
Two people were arrested by armed officers at the station.
Police have not identified the suspects or disclosed a motive, but said counter-terror police were supporting the investigation.
"Ten people have been taken to hospital with nine believed to have suffered life-threatening injuries," British Transport Police said in a statement early on Sunday.
"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."
The police force said "Plato" - the national code word used by police and emergency services when responding to what could be a "marauding terror attack" - was initiated.
That declaration was later rescinded but no motive for the attack was disclosed..
"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," 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 from Doncaster in northern England to London's King's Cross station headed south toward Huntingdon, a market town a few kilometres northwest of the university city of Cambridge.
Passenger Olly Foster told the BBC he heard people shouting "run, run, there's a guy literally stabbing everyone".
He said he initially thought it might have been a Halloween prank.
But as passengers pushed past him to get away, he noticed his hand was covered in blood from a chair he had leaned on.
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, and police officers wearing forensic suits, with a police dog, could be seen on the platform.
Cambridgeshire Constabulary, the local police force, said officers were at the scene, about 120km north of London at 7.39pm on Saturday UK time.
British Prime Minister Keir Starmer said his "thoughts are with all those affected" after the "appalling incident".
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 said there would be major disruption on the route until Monday.
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