A murderer is on the run in Surrey following the fatal stabbing of a train passenger in front of his 14-year-old son.

British Transport Police has told members of the public not to approach the suspect who killed a man, believed to be aged 51, on the 12.58 service travelling between Guildford and London Waterloo stations on Friday.

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Superintendent Paul Langley said: “What we know at the moment is that the victim boarded the train at around 1.00pm at London Road station in Guildford, along with his 14-year-old son.

“Traumatically, the victim’s son would have been nearby when his father was fatally wounded. This would have been an horrific and hugely traumatic event to have witnessed and we are providing him with as much support as possible.

“We know that the suspect also boarded the service at London Road station, however at this time we do not believe they are known to each other.”

He added: “We have a number of officers in the area working hard to trace the man responsible for this violent and deadly attack. We are working close with Surrey Police and the National Police Air Service who are helping us to locate him.

“The suspect is a black man, in his 20s to 30s, approximately 6ft tall and of slim build with a beard, believed to be dressed all in black with white trainers.

“At this moment I would ask the public to please contact police urgently by calling 999 if they see anyone matching this description – please do not approach him.”

Supt Langley said “violent events such as this on the rail network are extremely rare”, and there would be extra officers on duty on Friday evening and Saturday to help reassure concerned passengers and rail staff.

Police say the knifeman left the train at Clandon station and headed in the direction of Dedswell Drive.

A woman who lives locally reportedly told other villagers that she had spoken to a “very sweaty and nervous” man in a field near her home.

“Please be vigilant as I just came across a mid 20’s, slim build, black beard, bald suspicious man wearing a black tracksuit walking around our neighbouring field, very sweaty and nervous,” she is said to have posted on a neighbourhood watch website.

“He walk (sic) to the bottom then stood in the woodland, walk back up. I asked him what he was doing and he said he had an accident.

“Our road is private and so is the field. He walked off on the phone and didn’t want to chat. There was no reason for him to be there!”

Officers were called to Horsley station at 1.15pm to reports of a stabbing on a train.

The victim was said to have suffered “multiple stab wounds” in the “frightening” attack.

The South East Coast Ambulance Service confirmed the man died at the scene.

No arrests have been made in relation to the incident, police said.

Detective Superintendent Gary Richardson, from British Transport Police, said: “This was a shocking and violent attack which took place on board a train in broad daylight.

“Officers from BTP are with the family and have deployed a specially trained family liaison officer to support them through this difficult time.

“We know that this was an incredibly frightening incident for passengers travelling on the train.”

One social media user who said they had been on the train praised the “incredible” response of the guards and driver in a tweet to South Western Railway.

They wrote: “I was on this train, how those guards and especially that train driver went into action was nothing short of incredible. Not sure if it’s appropriate to say thank you, but I do hope everyone is ok after today’s event.”

A number of police officers and helicopters were deployed in the Horsley and Clandon area as a murder inquiry was launched.

Officers are urging any witnesses to the attack to get in touch.

Oliver Thompson, a 16-year-old student living in Clandon, said he was stopped by police in the area at roughly 2.30pm and asked if he had seen a “man running from the station”.

A little later he saw police helicopters circling the area before being joined by “a police van with high-powered lights” as conditions darkened.

A police officer aboard a train at Horsley station Credit: Steve Parsons/PA

“It comes as a big surprise as Clandon is a quiet area that you wouldn’t associate with crime like this,” he said.

“It is definitely unnerving to think there is a murderer maybe somewhere in the village.”

Transport Secretary Chris Grayling tweeted: “The incident near Horsley is tragic, and our deepest sympathies are with the victim and their family. I urge anyone who witnessed what happened to contact @BTP and @SurreyPolice, by texting 61016 or calling 0800 40 50 40.”

South Western Railway said the train line between Guildford and Effingham Junction towards London Waterloo was blocked for almost seven hours while police responded to the incident.

The train operator said all lines had been reopened with services returning to normal, but warned that some trains on the route may be delayed by up to 10 minutes or revised.

The disruption is expected to last until 11pm.

Replacement buses were earlier laid on for passengers and tickets were being accepted on some Southern Railway and Stagecoach bus services.