Rail crash investigators are trying to establish why the Potters Bar railway tragedy which has so far claimed seven lives happened.

Faulty points near the railway station have been pinpointed as the cause of the disaster.

But the question remains as to why the points - the mechanism which divert trains onto different tracks - were not functioning properly.

There are various theories, ranging from deliberate vandalism to poor maintenance or inadequate inspections.

John Armitt, the chief executive of rail operator Railtrack, says vital nuts were missing from key bolts in the points structure.

The rear carriage flipped across two platforms

Their absence put too much stress on a load-bearing track at the junction, causing it to fracture as the doomed 1245 service from London to King's Lynn passenger express passed over at high speed.

The company responsible for maintaining the track, Jarvis plc, insists it had checked that section of the track, only the day before.

There is other speculation.

The Observer newspaper said unnamed senior rail industry sources had identified a "sophisticated" act of deliberate damage as a possible cause of the fault.

But the proximity of the faulty track to the busy Potters Bar station would have made sabotage difficult.

'Getting safer'

The Independent on Sunday, meanwhile, quoted Bob Crow, general secretary of the rail union RMT, saying that there had been poor maintenance and inadequate inspections which could have led to problems with the nuts.

If that theory is confirmed, it would plunge the rail network into renewed turmoil just as it was emerging from the devastation of the Hatfield crash in October 2000.

That accident, when four people were killed after an express derailed, was just a few miles from Potters Bar on the same line.

It was blamed on a cracked rail, forcing Railtrack into its most extensive track overhaul for half a century.

But UK Transport Secretary Stephen Byers insists the railways are still a safe way to travel despite concern at the death toll since privatisation.

"If you look at the trend, then rail travel per passenger mile is actually getting safer.

"We need to do more, there are no grounds for complacency. All the measures are being put in place to ensure it is as safe as it possibly can be," he said.