Ten people are in hospital - nine believed to have life-threatening injuries - after a stabbing attack on board a Doncaster-London King's Cross train on Saturday.
Two people have been arrested after the train made an unscheduled stop at Huntingdon station, where dozens of officers rushed to help.
Police received calls from passengers on board the service at about 19:40, alerting them to the incident.
British Transport Police say counter-terrorism officers are supporting the investigation as they "work to establish the full circumstances and motivation".
An eyewitness tells the BBC they saw a man with a bloodied arm fleeing down a carriage yelling "they've got a knife" - another witness says police Tasered a man on a platform.
"All the passengers along the train were just packing forwards... all huddled together," Wren Chambers tells BBC.
An eyewitness who was on the train at the time of the incident says passengers were left in "pure panic" as bleeding victims called out for help.
Olly Foster told the BBC he initially heard people shouting "run, run, there's a guy literally stabbing everyone", and believed it might have been a night-after-Halloween prank.
Within minutes, people started pushing through the carriage, and Foster noticed his hand was "covered in blood" as there was "blood all over the chair" he had leaned on.
An older man "blocked" the attacker from stabbing a younger girl, leaving him with a gash on his head and neck, Foster said. Passengers around him used jackets to try to staunch the bleeding.
He added that the only thing people in his carriage could use against the attacker was a bottle of Jack Daniel's whiskey, leaving them "praying" that he would not enter the carriage.
Although it lasted 10 to 15 minutes in total, Foster says the incident "felt like forever".
Prime Minister Keir Starmer says the "appalling incident" is "deeply concerning" and urges people to follow the advice of local authorities.
London North Eastern Railway (LNER) has urged passengers to defer rail travel, and says disruption is expected until the end of the day on Sunday.
In a statement published early on Sunday morning, LNER Managing Director David Horne wrote that he was "deeply shocked and saddened by this serious incident" and thanked emergency services for their "quick and professional" response.
"We will continue to do everything we can to support our customers and colleagues during this difficult time," he said, reiterating that the wellbeing of everyone affected would remain his top priority.
Witnesses who were on board the train at the time of the incident described the horrific scenes to the media.
Passengers told Sky News that the stabbing started 10 minutes after the train left Peterborough, and that wounded people were seen running through the train away from somebody with a knife.
A man holding a large knife was later seen on the train platform with armed police pointing their weapons at him, one witness added - saying the man was then tasered and restrained.
Several MPs have described the scene in Huntingdon as "horrifying".
Shadow Home Secretary Chris Philp wrote on X that his thoughts were with "all those injured and affected", and called on the police and the government to provide an update on the incident.
Conservative Kevin Hollinrake MP echoed his words, writing that his thoughts were with the victims following the "horrifying scenes in Huntingdon".
Knife crime in the UK has "steadily" risen since 2011, according to government statistics.
It's become such a prominent issue that Sir Keir Starmer has branded it a "national crisis'.
Sixty thousand knives have been either handed over or seized by police as part of a broader government crackdown over the past year. Carrying a knife in public is punishable by up to four years imprisonment.
Last night's Huntingdon train stabbing and last month's knife attack at a Manchester synagogue, which left two dead, have both made international headlines.
