The points, at which the train derailed, killing seven and injuring about 40, are being cut out and sent to the Health and Safety Executive's laboratory for a forensic examination.

A set of nuts at the points have already been removed and are being inspected by the HSE.

Experts say loose nuts caused a "stretcher bar" at the points to fracture

The points, the mechanisms which divert trains onto different tracks, contain three "stretcher bars" which keep the tracks in place as the train passes over them.

Each of these stretcher bars is held in turn by four anti-vibration nuts. At Potters Bar it is believed at least two sets of nuts were missing - one on each of the second two stretcher bars.

This meant they were not fully bolted on and did not share the pressure as the train passed. The subsequent pressure on the first stretcher bar caused it to fracture, sending the fourth carriage off the rails.

'Vandalism' theory

But the question remains as to why the points were not functioning properly.

They were checked just the day before the crash, according to Jarvis, the company responsible for maintaining the track at the scene.

	Potters Bar crash Seven people died Two are still critically injured Investigation teams are combing the scene

Transport Secretary Stephen Byers told the Commons on Monday Railtrack had subsequently inspected 800 sets of points across the country and found no similar defects.

At the weekend there was speculation that vandals were to blame.

But this theory became increasingly unlikely as it emerged that specialist equipment would have been needed to remove the nuts.

It would also have to have been done in full view of the station, which has CCTV, on a live track.

More likely, experts are now concluding, was that poor maintenance played a role.

Rail expert Richard Hope, of Railway Gazette magazine believed the problem may have been caused by inexperienced workers.

"It may be that they were taken off during an inspection and somebody forgot to put them back on. Or somebody didn't tighten them up properly and they were shaken loose."

Inspections

Railtrack has admitted that it carried out work on that section of track, including the points, last year.

But it said the work was to do with "gauge corner cracking" which it and other experts agreed was not an issue in the Potters Bar accident.

Points inspections

1 May: Tests and inspections 9 May: Visual inspection All inspections by fully-qualified staff Tests to recognised standards Source: Jarvis

Jarvis said the track had been subject to two recent checks - tests and inspections on 1 May, and visual checks on 9 May.

"In both cases, these tests and inspections were carried out by full-time Jarvis employees. No sub-contracted personnel were involved."

It added that tests were carried out in accordance to standards recognised by the Institute of Railway Signalling Engineers.

However, a Jarvis employee contacted BBC News 24 to say workers were under pressure to "cut corners".

He claimed the track was checked less frequently than it was under British Rail, and said safety was being sacrificed for profit.

Experts are mystified as to why the nuts were missing

The rail union RMT said a worker had alerted management three weeks ago to the fact that bolts were missing at the points, and that the track in that area was generally in a poor state - but had received no response.

Union leader Bob Crow said rust and a lack of maintenance probably caused the accident: "I think the vibrations of the train allowed the nuts to come off the bolts."

Mr Crow and others said there was a problem with the complex structure of responsibility for maintenance work.

Railtrack bears responsibility for the work, but it farms it out to companies such as Jarvis, who in turn subcontract out work to hundreds of different firms.

All report to the HSE, but there are concerns than lines of responsibility have become blurred.

Mr Crow said safety standards were not high enough because there were not enough track inspectors, and too many casual workers.

Labour's George Stevenson MP, of the Transport Select Committee, said there was a simple answer.

"The government must now take the next logical step, and take into public control all the railway infrastructure."