British Transport Police (BTP) have set up a special hotline so rail workers with information on the Potters Bar crash can contact them in confidence.

Detectives say they still do not have "the full picture" of the accident in May this year, and are urging rail and maintenance staff to call in.

We are not saying that people are deliberately hiding anything

Det Supt Paul Crowther

They believe workers in the industry may feel constrained by peer pressure or other factors, and could be reluctant to be seen speaking to police.

Seven people died and 76 were injured when a West Anglia Great Northern train derailed at a faulty set of points just outside the station at Potters Bar in Hertfordshire.

Inside knowledge

A report into the crash published by the Health and Safety Executive in July said that loose nuts on parts of the track caused rail points "to fail catastrophically".

Detective Superintendent Paul Crowther, who is leading the inquiry - Operation Declare - said: "We feel that there are people in the rail industry with knowledge of the work and working practices in the Potters Bar area who still have useful information to give us."

Potters Bar hotline

0207 887 6864

He went on: "We are getting good co-operation from the companies involved, but it is inevitable that some staff feel constrained."

Mr Crowther said he did not believe workers were deliberately hiding any information related to the crash.

"But peer pressure, anxiety and a reluctance to be seen talking to the police can be significant factors in impeding the free flow of information," he said.

Rail maintenance company Jarvis was responsible for maintaining the section of track at Potters Bar.

In the immediate aftermath of the crash, the company said sabotage could not be ruled out and claimed it had evidence that nuts had been adjusted on the points in the two days before the crash.

However no official maintenance had been carried out during that period.

Last month's HSE report concluded that points were not up to standard and nuts on other points in the area were found not to be fully tightened.

Study of footage

In June, the BTP issued an appeal for five "mystery" railway workers seen on the track hours before the train crash.

Chief Superintendent David Hatcher told BBC News Online on Wednesday that the workers had yet to come forward.

The five, wearing orange rail worker jackets, were captured on CCTV and seen by other workers at 0825 BST - more than four hours before the crash.

No-one from Railtrack or Jarvis appears to know who they are.

Mr Hatcher added: "We have now sent the video tape to the FBI for enhancement so we might get a clearer picture of who they are."

A team of 50 officers is working on Operation Declare and has questioned over 2,000 people, and taken over 350 statements.