The maintenance record for the track near Potters Bar is being examined after it was confirmed nuts were off the rails nine days before the fatal crash.

The Health and Safety Executive has released an interim report confirming that missing rail nuts in a set of points probably caused the derailment, which killed seven and injured 76.

Rail maintenance contractor Jarvis said the nuts were found detached and screwed back in place during a routine check on 1 May.

A further visual check on 9 May apparently found nothing wrong.

But the HSE said the nuts were off again by 10 May, causing the points to break and the train, in turn, to derail.

The HSE wants to establish why the nuts were missing
	Points are held in place by 'stretcher bars' Each bar held by four anti-vibration nuts Why were nuts missing at Potters Bar?  Click here to find out more about the Potters Bar points and how they should work

The HSE said the Jarvis revelation was "very significant" and that it was examining all four detached nuts in its laboratory. It said British Transport Police were also examining some components.

On Tuesday afternoon the train carriage wedged beneath a platform roof at Potters Bar station was lifted clear by crane.

It was placed on a specialist lorry, where it will be searched by police and the HSE, before being handed back to operator WAGN.

Work is continuing to remove a 45-metre section of railway track and points nearby, which will also be taken to an HSE laboratory.

Railtrack said it hoped to have services running through the station again by Monday of next week.

'Basic error'

As rail critics began to blame maintenance errors for the accident, the HSE said all the maintenance records of the area over the last 12 months would be thoroughly examined.

These nuts have never, ever come off except to have been manually removed

Jarvis

Frank Hyland, who is managing the HSE's investigation, said: "The question is 'why did we find nuts missing or not in position?'.

"We shall be working with British Transport Police to build up a detailed history of all the points in the area."

He would not comment further on the state or position of the nuts.

A Jarvis spokesman said: "These nuts have never, ever been discovered to have fallen off.

"They have never, ever come off except to have been manually removed."

Jarvis and Railtrack both said they had not ruled out sabotage.

Later Transport Secretary Stephen Byers released a letter from Jarvis which said: "The nuts were refitted and retightened in accordance with good industry practice."

A former rail engineer who did not want to be named told BBC News there was a "lack of knowledge" in all aspects of track maintenance work.

And rail expert Christian Wolmar blamed the accident on "a basic error".

Contracting-out

In a separate development, it emerged on Tuesday that vandals tried to derail a passenger train 24 hours after the Potters Bar disaster by putting football goalposts across a track near Oxford.

The HSE said it had found no evidence of vandalism or signal problems at Potters Bar, and that the train was being driven normally.

The report said the train coaches had generally stood up well to the impact, as had the railway bridge and the station buildings.

 Click here to read the report's main points

	:	DIAGRAM
Click here for a detailed diagram of the points

The investigation has also thrown the spotlight on the culture of contracting out work on the railways.

Prime Minister Tony Blair told BBC's Newsnight "it's going to take a lot of time and money over many years" to get the railways right - but he backed Mr Byers to do it.

Gwyneth Dunwoody, head of the Commons Transport Committee, called for Railtrack to take maintenance back into its own hands and out of that of contractors.

She said: "There is a lot of evidence that people have been taken on when they have not got the proper classifications."

Mick Rix, general secretary of the train drivers' union Aslef, said: "The conduct and culture of infrastructure maintenance should now be reviewed as a matter of the highest priority."

But Dr Allan Sefton, the HSE's acting chief inspector of railways, said: "There is no evidence at the moment that this is a generic problem."