He had tried to intervene after overhearing a young woman in the carriage being verbally abused by a man, believed to be her ex-boyfriend.

The teenager, an undergraduate who was in his first year at St Andrews University, died from a single wound to the chest shortly after noon on Saturday.

Mr Grant, believed to be an only child, had been travelling back alone to his family home after sitting an exam. It is understood he had used the west coast route because he wanted to take his bike on the train.

Other passengers in the carriage fled after the attack, and the train manager was able to lock the killer in the carriage; but the man kicked in a window and fled as the train, which was travelling from Glasgow to Paignton, Devon, pulled into a station in Cumbria.

His escape prompted an extensive manhunt, involving armed police, trained dogs and a helicopter. A 21-year-old man was later arrested in the Kendal area and was immediately taken for questioning by police.

The man, from Skelmersdale, West Lancashire was last night charged with murder. Police have not yet named him.

Passengers who witnessed the killing and were caught up in the panic that followed were said to have been traumatised by the incident.

Details of the killing, on board the 10.10am Virgin Trains service from Glasgow, emerged yesterday as shocked passengers spoke of the violence they witnessed.

The train was travelling south at up to 125mph when a couple who were travelling with three young children became involved in a heated argument. As the row escalated, Mr Grant, who was sitting near by with a woman friend, intervened in an attempt to act as a peacemaker.

Witnesses said that his attacker became highly agitated and began pacing up and down the aisle of the carriage before suddenly lunging at Mr Grant, stabbing him in the chest with a knife. The student collapsed and died before paramedics were able to tend to him.

A police source said he had been carrying out an “essential gentlemanly act”.

Passengers alerted rail staff and the train manager took swift action to isolate the killer. He quickly evacuated the carriage, moving about 20 passengers to an adjoining carriage before disabling the electric doors and trapping the knifeman, alone, inside.

Police and paramedics were alerted, but when the train slowed down in the approach to its scheduled stop at Oxenholme, the man used his foot to smash one of the emergency windows.

A number of people waiting on the platform saw him climb out of the moving train before jumping on to the tracks. He ran into a field and escaped, despite several members of the public pursuing him.

Josephine Whittaker, who was on the platform as the train pulled in, described what she had seen. She said: “He actually kicked out a window and it came out in one stroke, as those windows are meant to do in an emergency.”

The train, with 200 passengers on board, was held at the station for three hours, causing long delays on the West Coast Main Line as witnesses were questioned by police.

Crime scene analysts were studying the carriage yesterday, while a post-mortem examination on Mr Grant was conducted by a Home Office pathologist.

Counselling was offered to passengers who had witnessed the killing, which happened in one of the train’s rear carriages. Some of the onlookers were treated by paramedics for shock.

The Government announced a nationwide knife amnesty last week in an attempt to reduce the escalating number of stabbing incidents.

British Transport Police announced last month that it was extending the use of metal scanners at train stations as part of a move to cut knife crime. The scanners are not in use in Scotland, where the Virgin train began its journey.

GETTING TOUGH

• Attacks by strangers are now the largest component of overall violent crime in England and Wales

• Stranger violence made up 35 per cent of violent crime in 2004-05 compared with 24 per cent in 1995

• In 1995 acquaintance violence was 43 per cent compared with 34 per cent in 2004-05, according to the British Crime Survey

• Almost 30 per cent of homicides involve a knife

• There were 1,200 crimes involving knives in London last year

• Men aged 16-24 are at the most risk

• There is a maximum two-years sentence for carrying a knife in public without good reason and a maximum five years in jail for carrying an offensive weapon, such as a flick knife

• John Reid is to consider a minimum sentence for carrying knives in public

• A five-week knife amnesty started last week

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