Forensic tests are being carried out to find out why nuts on a points system outside Potters Bar station loosened and caused the train derailment that claimed seven lives.

Railtrack said on Saturday that the vital components in the trackwork had detached, forcing the rear carriage off the rails as the train passed over on Friday.

The company responsible for maintaining the track at the scene has confirmed the points passed a safety inspection only the day before.

Meanwhile, police are still trying to identity one of the dead and one of the crash survivors who is in a critical condition in hospital.

The rear carriage flipped across two platforms
	Potters Bar crash Seven people died Two are still critically injured Investigation teams are combing the scene More images

The injured woman, whose age has not been released, was taken to the Royal Free Hospital in Hampstead, north London, with head injuries.

Remembrance services are being held in Potters Bar on Sunday, after floral tributes were laid at the scene for a second night.

As work continues to clear up the crash site, investigators are to start a minute forensic examination of the failed points.

One element of the inquiry will involve looking for rust in the bolt holes in an effort to date how long they had been dislodged for.

Speculation that vandals are to blame increased after the engineering firm in charge of maintenance on the East Coast Main Line, Jarvis PLC, backed claims by Railtrack that the points were checked on Thursday.

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

Vandals

"This particular site was last inspected on May 9 and the points in question were compliant with standards at the time of this inspection."

Earlier John Armitt, Railtrack chief executive, said the points were "visually inspected" the day before and examined more thoroughly on 1 May.

He added: "We are unclear why nuts which were an integral part of the points were detached, and the investigation will look into this with great detail.

"We will not rest until we have exhausted the investigation."

Byers insists railways are still safe

Vandalism may have caused the damage, but experts also say the bolts could have been weakened by passing trains.

A maintenance failure could also be to blame, but a nationwide inspection of points around the network has so far revealed no similar problems.

The accident happened just before 1300 BST on Friday when the rear carriage of a WAGN train derailed and flipped across two platforms before wedging under the canopy of the Hertfordshire station's roof.

Dead named

The carriage will be removed by a special 11,000 ton crane later on Sunday, and trains are being diverted via Hertford.

Earlier in the day, Transport Secretary Stephen Byers visited the site and moved to reassure the public that rail travel was "still one of the safest forms of transport".

He said: "It's very difficult in a situation like this to say that the network is safe but if you compare the network to other forms of travel it is one of the safest forms."

Railtrack boss John Armitt: The crash was a "savage blow"

Also on Saturday, Prince Charles visited survivors of the crash in Barnet General Hospital and Chase Farm Hospital in Enfield.

Police have confirmed that the former head of the BBC World Service, Austen Kark, was among the seven killed in the disaster.

Eighty-year-old Agnes Quinlivan - a great-grandmother of two and grandmother of eight from Potters Bar - has also been named as being among the dead.

Officers have confirmed the names of four others but are still trying to identify the seventh victim who they say is a foreign woman.

Two people remain in a critical condition in hospital.

Emergency information phone number: 0845 944 1551. For national rail inquiries call 08457 48 49 50.