Video report by ITV News Correspondent Martha Fairlie
A suspect is on the run after a man was stabbed to death on board a train in front of his 14-year-old son.
The 51-year-old man died at the scene after receiving multiple stab wounds in a "shocking and violent attack".
The victim and his son had boarded the train at Guildford London Road station. The suspect also got on the train there, but the two men are not believed to have known each other.
The police have launched a manhunt after the suspect fled the scene at Clandon, the next stop on from Guildford.
A number of police officers remain in the Clandon area where several sightings of the suspect have been reported. He is described as a black man of slim build in his 20s or 30s wearing all black with white trainers.
Officers and ambulance crews were called to the scene at Horsley station in Surrey at just after 1pm on Friday afternoon after reports a man had been stabbed on board the 12.58pm train service travelling between Guildford and London Waterloo.
Police have appealed for anyone who was on the train at the time of the attack, whether they saw anything or not, to come forward.
In a statement Superintendent Paul Langley of British Transport Police said BTP: "Tragically as a result of this stabbing, a man believed to be 51-years-old was pronounced dead on board the train.
"Early indications suggest he received multiple stab wounds to his body. What we know at the moment is that the victim boarded the train at around 1pm 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."
He warned people not to approach the suspect.
Earlier, 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."
No arrests have been made at present and the suspect remains on the run.
John Tindall who runs the cycle shop next to the station, said about 20 or 30 passengers "calmly" got off the train.
"There was no panic at all, there was just people calmly getting off the train and just wandering around. That's why I didn't think much of it really."
Mr Tindall said it was not a typical incident for the area, describing it as a "bit unusual".
Anyone who witnessed the incident or with any information should contact BTP by sending a text to 61016 or by calling 0800 40 50 40 – quoting reference 210 of 04/01/19. Or you can ring Crimestoppers anonymously on 0800 555 111.
The train line to Waterloo is currently blocked, with services running through stations between Guildford and Effingham Junction likely to cancelled, delayed or revised, South Western Railway said.
Disruption is expected until 7pm on Friday evening.
Members of the public who said they were on the train praised the response of the driver and the guards to the incident, with one woman on Twitter saying it was "nothing short of incredible".
Transport secretary Chris Grayling said in a statement on Twitter: “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.”