The deadly secret of Points 2182

A catastrophic failure of a set of points just south of Potters Bar station was named last night as the cause of the crash which killed seven people and injured many more on Friday.

Dr Alan Sefton, acting chief inspector of railways, said: "We are already becoming pretty clear on the initial cause of the crash ­ the fact that the points did fail under this train."

Railtrack engineers were reported last night to be checking points across the national network. Initial inspection of 400 points found them to be safe.

Stephen Byers, the Transport Secretary, visited the crash site and assured passengers the railway system was safe, as forensic experts sifted the evidence to pinpoint the cause of the accident.

Attention was focusing on loosened bolts on Points 2182, which appeared to change as the 12.45 King's Cross to King's Lynn train was passing over them at 100mph. Investigators were working through the night to find out how the bolts became loose. Eighteen months ago a train was derailed at Hatfield, four miles down the line, killing four people, because of a cracked rail.

Bob Crow, general secretary of the RMT rail union, said there had been poor maintenance and inadequate inspections which could have led to problems with the bolts, sending the last carriage careering into the platform at Potters Bar and shearing off its wheels.

Another theory was sabotage and there were suggestions that the bolts could have been tampered with by vandals. A senior investigator confirmed last night that bolts holding the two sections of rail together were "not as they should be".

Confirmation that points failure was to blame threatens to plunge Britain's rail network into a fresh crisis just as it was emerging from the devastation caused by a cracked rail at the Hatfield crash.

The embattled Mr Byers will make a statement to the Commons tomorrow. He is certain to face searching questions about claims by union leaders that poor maintenance and inadequate rail inspections lay behind the crash.

Last night the first of the dead were named as Agnes Quinlivan, 80, a great-grandmother from Potters Bar; Jonael Schickler, a 25-year-old German student at Cambridge University; and Austen Kark, 75, the former head of the BBC World Service.

Mr Kark was travelling with his wife, the author, Nina Bawden, 77, who last night remained in Barnet General Hospital with a fractured collarbone and ankle. Her injuries were not believed to be life-threatening. The Prince of Wales visited the injured in Barnet and at Chase Farm Hospital, Enfield. Relatives laid flowers at the crash site and a memorial service will be held today at St Mary's Church, Potters Bar.

As the grim task of clearing up the debris from the crash began, ministers were bracing themselves for the political fall-out. There were renewed calls by Labour MPs for the Government to renationalise the rail network, a view supported by the public in internet chat sites that sprang up after the crash.

The Conservatives and Liberal Democrats, meanwhile, softened their calls for Mr Byers to be sacked, but ministerial colleagues said Tony Blair may be forced to drop the Transport Secretary in the July reshuffle.

Other theories were still being considered yesterday. For instance, a wheel may have fractured due to metal fatigue, causing the points to switch as the train passed. This was the cause of Europe's worst rail accident, which occurred on the Hanover-to-Hamburg express in 1998. More than 100 people were killed.

Mr Crow told The Independent on Sunday that maintenance workers had repeatedly warned Railtrack about metal fatigue and flooding on the line.

His union claims that a loosened bolt was missed because the last overhaul of the points was carried out in January, rather than every six weeks. Denying the charge, Railtrack's chief executive, John Armitt, said the points had been inspected in the last month.