'Heroic' staffer fighting for his life after saving people in UK train stabbing

About 24 hours ago, bloody chaos erupted on a London-bound train.

A rail worker is being hailed as a hero as he fights for his life in hospital after a horrific mass stabbing on a UK train put 11 people in hospital.

Police said the man tried to stop the attack and "saved lives" by his actions.

The staff member was one of 11 people hospitalised after the mass stabbing, and is the only one in a critical condition.

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Police have yet to lay charges over the incident, though one man remains in custody.

Here's what we do know about the attack.

What happened?

At about 7.42pm Saturday night local time (6.42 am Sunday AEDT) passengers aboard a London-bound train from Doncaster made the first emergency calls to police, with reports of people fleeing through the carriages and seeking shelter in the train toilets.

The train driver, since identified as a war veteran, pulled the train in to an emergency stop at Huntingdon, about 120km north of London.

Police swarmed the train and arrested two men at about 7.50pm local time, just eight minutes after the first emergency calls.

Ten people were taken to hospital, while an 11th later self-reported.

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Was it a terror attack?

In their initial response, police treated the mass stabbing as a potential terror incident, broadcasting the code-word "Plato" to responding officers - the term used to indicate a "marauding terror attack".

However, authorities have since walked that back, without speculating further as to motive.

"This is a shocking incident and my thoughts are with those who have been injured and their families," British Transport Police Superintendent John Loveless said outside the station in Huntingdon in eastern England where the train halted soon after the attack.

"There is nothing to suggest this is a terrorist incident."

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How are the victims?

Six people remain in hospital as of this morning (AEDT), with one in a critical condition.

That person has been identified as a railway staffer who tried to stop the attack, and who police say saved lives with his "heroic" actions.

"Having viewed the CCTV from the train, the actions of the member of rail staff were nothing short of heroic and undoubtedly saved people's lives," Deputy Chief Constable Stuart Cundy said.

Five other people have been discharged from hospital.

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Who were the men arrested?

A 32-year-old man and a 35-year-old man were both taken into custody at the scene.

However, this morning, police confirmed they had released the 35-year-old man without charge, saying he had not been involved.

"It was reported in good faith to officers responding to the incident that he was involved in the attack, and following enquiries we can confirm that he was not involved," police said in a release.

The 32-year-old remains in custody.

He has been identified as a British national from Peterbough, which is where police say he boarded the train.

A knife has also been recovered at the scene.

What happens next?

It is likely the sole remaining suspect will be charged once police have finished their interrogation.

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 local time (Monday evening/overnight AEDT).

Loveless said passengers will see a "high visibility presence of police officers at stations and on trains" on Sunday.

Reported with additional details from Associated Press.

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