It has been confirmed that missing nuts at a set of points were probably to blame for the Potters Bar crash. How do points work, what tests were carried out on them, and why were the nuts missing?

What exactly happened?

The train was derailed at Potters Bar because of missing nuts at a set of points - the mechanisms that divert trains onto different tracks.

Points contain two "stretcher bars", held in place by a set of nuts, which support the tracks and keep them in place as the train passes over them.

	:	DIAGRAM
Click here for a detailed diagram of the points

At Potters Bar two sets of nuts were missing - one from each of the bars.

The bars came loose and were unable to share the pressure caused by the passing train.

The subsequent massive pressure on the points' locking bar caused it to fracture, allowing the fourth carriage of the train to be deflected to the left, before derailing and skidding along the track.

Could this happen elsewhere?

Transport Secretary Stephen Byers described the accident as a "one-off".

Railtrack checked about 800 sets of points nationwide and found no similar defects.

The Health and Safety Executive checked points in the Potters Bar area and found no similar defects that required speed, or any other restrictions elsewhere on the network.

Had there been previous signs of a problem?

Jarvis, the contractor responsible for that length of rail, checked the tracks twice in the days before the accident.

It said tests and inspections on 1 May found a loose pair of nuts, which were replaced.

Yet a visual check on 9 May, the day before the accident, apparently found nothing wrong.

Points inspections

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

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

In March, commuter Kevin O'Neill raised concerns about the same section in a letter to the HSE warning that a passenger had said: "If that is not a derailment waiting to happen, then I don't know what is."

But Railtrack chief executive John Armitt says he has no record of these warnings being given.

Were the points in a poor state?

The points were eight years old, and the natural lifespan of points is normally 20 to 25 years.

Bob Crow of RMT believes the nuts came off because of rust and other problems of disrepair, which developed through too few track inspections.

There were some loose ballast on the check rail which guides the train into the points, and some bolts were seized with rust

Potters Bar rail worker

The rail worker who sent the warning - who did not wish to be named - said there were several problems with that part of track.

"There were some loose ballast on the check rail which guides the train into the points, and some bolts were seized with rust."

The HSE would not comment on a suggestion that the nuts were in such a position that they could have just fallen off - but it said this was being looked into.

Could the nuts have been removed - by accident or design?

An early theory of deliberate vandalism has now largely been discounted.

The focus is now on the adequacy of the maintenance conducted.

Experts are mystified as to why the nuts were missing

Mr Hope believes inexperienced workers may have been to blame for the missing nuts.

"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."

But Jarvis said checks were carried out by full-time, fully-qualified employees.

"The signalling maintenance team were Jarvis operatives licensed to standards recognised by the Institute of Railway Signalling Engineers.

"Our extensive records show that all maintenance work in the area has been carried out fully in accordance with group and line standards."

Were checks adequate?

Rail expert Christian Wolmar, author of Broken Rail, said he understood once-a-week points inspections were "normal" across the industry.

He said the inspections should have picked up any problem, and blamed the accident on "a basic error in maintenance".

"This is another maintenance failure, after the Hatfield crash ... only five miles up the track.

"You would think that the maintenance contractors in that area would be especially careful not to be making a mistake, and yet they have clearly made one. "

But rail expert Richard Hope, of Railway Gazette magazine, said the 9 May visual inspection could easily have failed to spot missing nuts.

"They would look over the points but I don't think they would be expected to touch anything. They wouldn't necessarily see if the nuts weren't there."

Does anybody monitor the checks?

A spokeswoman said Railtrack audited Jarvis's checks "fairly regularly" - more than once a year.

The audits took the form of checking paperwork that Jarvis had submitted, she said.

But Mr Wolmar said the HSE had for some time been worried about the monitoring of contractors.

"A lot of the work is self-certified and it's tendered on that basis, which is fine if the contractor is reputable... but the HSE had extreme concerns at the time [of the Hatfield crash in October 2000] and it is still a concern."

What will happen now?

The nuts and part of the points have been taken away for forensic examination by the HSE.

It published an interim report on Tuesday - confirming the fault in the points - and is conducting a fuller formal investigation.

British Transport Police also have some components and are studying track maintenance records.

The Conservatives and RMT are calling for a public inquiry into the crash and the whole issue of rail safety management.