Vandalism? A bomb? A defective wheel? The truth lies in the wreckage of the 12.10 to Leeds

The investigation

Tensions rose last night between police and rail industry investigators after inspectors from the Health and Safety Executive were banished from the site of the rail crash.

Tensions rose last night between police and rail industry investigators after inspectors from the Health and Safety Executive were banished from the site of the rail crash.

Police said they had a duty to investigate a suspicion that a terrorist device might have caused the derailment. Inspectors, however, were considering other possibilities and expressing private frustration that they were not able to start their work.

Rail experts believed the derailment had been caused by a defective wheel or axle, vandalism or a broken rail. The police, however, confirmed last night that a bomb threat had been made against the King's Cross to Peterborough line two days earlier.

Speaking near the scene of the derailment, in which four people died, Assistant Chief Constable Paul Nicholas, of the British Transport Police, said that because of the threat police could not rule out terrorism. However, he accepted that the injuries sustained by the victims were not consistent with a bomb blast. "Threats are made on a daily basis but very rarely end up with an incident," he conceded.

Asked for his comments on reports of a bang before the crash, Asst Chief Constable Nicholas said: "It is not unusual for a train to make an explosive sound when it breaks apart - comes apart from its couplings."

He would not confirm if a coded warning had been given.

Peter Rayner, adviser to the Commons committee on transport, registered scepticism that the derailment had been caused by terrorism. He urged the police to allow rail inspectors back on to the disaster site. "They should get off the site and allow the rail engineers and other people who know what they are doing to find out the cause of the crash," he said.

Far more likely than terrorism, according to rail experts, was a catastrophic failure of one of the wheels or axles on a coach towards the front of the train. Both incidents are rare but extremely dangerous.

One of the last instances was on the same East Coast line as yesterday's accident and it affected the same kind of rolling stock. Nine passengers suffered minor injuries at Sandy in Bedfordshire on 16 June 1998 when part of the nine-coach 5.30pm from King's Cross to Edinburgh service came off the track. In July this year, Railcare Ltd was fined £175,000 and ordered to pay £30,715 costs over the incident in which a wheel on the train was said to have disintegrated.

The company removed all Mark 4 coaches from service so that the wheels could be checked after discussions with Railtrack and the Health and Safety Executive. A spokesman for Great North Eastern Railway said last night that all the wheels on the rolling stock had been changed.

Investigations found metal fatigue in the wheels of the rolling stock, which is operated only by GNER and which was introduced in 1989.

Vandalism was also thought to be a possible cause of yesterday's tragedy. However, the fact that the rear carriages of the GNER train were derailed rather than the front ones led rail experts to be sceptical.

The main-line track between London and Leeds has been one of the worst areas on the network for trespass and vandalism. Railtrack's London North Eastern area, which covers the site of yesterday's crash, has reported nearly 1,000 incidents of trespass and more than 1,500 cases of vandalism.

As part of "Operation Skyhawk", the infrastructure company has used helicopters to catch intruders, and in "Operation Scarecrow" railway staff and British Transport Police have patrolled the worstaffected areas.

The network has also been plagued by broken rails - another possible cause of yesterday's crash. Railtrack sources pointed out that the section of track on which the derailment happened was continuously welded, which would drastically decrease the chances of a crack appearing. Jonathan Cox, Railtrack's director of operations, said the track was inspected by safety experts on Monday - the second inspection on the line between London and Leeds in a week.

There were 949 instances of broken rails on the railway in 1999-2000, slightly down on the previous year's figure of 988 but well up on the totals for the three years from 1995 to 1998. The number of buckled rails rose from 21 in 1998-99 to 64 in 1999-2000.

The annual number of what the Health and Safety Executive describes as "significant" passenger train derailments has remained fairly constant in recent years at about 22 to 29.

Pip Dunn, of RaiI magazine, pointed out that broken wheels and broken axles were "fantastically rare" on the railways, "but when they do happen they can be catastrophic".

He said: "It was very fortunate that the incident at Sandy two years ago did not result in any serious injuries.

"Although vandalism can never be discounted, it appears to be unlikely in this incident and the fact that the stretch of track was continuously welded also suggests that a broken rail is not the cause, either."

Chris Milner, deputy editor of Railway Magazine, said inspectors would examine whether there was vandalism, track failure through breakage, buckling or wheel/axle failure.

"From what we know of the incident it was the rear part which was derailed.

"I would have thought that if any debris was placed on the line, it would have either been pushed away or caused the front part of the train to derail.

"That suggests a failure in the train's wheels or axles, or a track defect, and that will be for the investigators to look into," he said.