The train and track are being inspected

A Railtrack spokesman said: "We are looking at the train, the infrastructure and outside influences such as trespass and vandalism."

The train had been diverted because of routine maintenance work by Railtrack which had caused the closure of a section of the West Coast main Line.

No-one was seriously injured, although two passengers suffered shock, one received a slight neck injury and one was reported to have received cuts and bruises.

Confidence 'low'

A transport police spokesman said that vandalism had been "all but ruled out".

Earlier, Professor David Begg, director of the Centre for Transport Policy in Aberdeen, said that investigations would focus on vandalism or problems with the points.

He said: "When the middle carriages come off they will look much more closely at a broken rail, but where it's the front carriages the probability of vandalism is higher, but also problems with the points."

The area remains sealed off

The train, the 1030GMT service from Euston, had already been delayed by more than two hours after three diversions on its way from London.

Railtrack Scotland infrastructure manager Jim Bellingham said: "Although this was a slow-moving derailment with the carriages remaining upright, we do treat this very seriously and are quite saddened that this has happened in Scotland."

He said that Railtrack understood that public confidence was low because of recent incidents.

David Grundy, 39, who was travelling from Bolton, Greater Manchester, to Glasgow, said the incident was "like an earthquake".

Luggage 'flew'

He said: "It was shaking all over the place. Everyone was rushing about. There was luggage flying about.

"It shook and shook and then it stopped, and there were people crying for help."

Passengers were kept on the train for nearly two hours while the police took details. They were then taken to a local community centre.

Journeys between London and Glasgow currently take up to nine hours because of speed restrictions, compared to five hours in normal circumstances.

A spokeswoman for the Health and Safety Executive said it expected to be able to clear the train from the tracks in the next few days.

The Hatfield crash, which killed four people and injured 35, was caused when a broken rail derailed a GNER London to Leeds service.

Since the accident, on 17 October, a huge programme of rerailing has been taking place across the UK rail network.