Seven people died in the Potters Bar rail crash in May 2002

The train derailed after passing over points where nuts were missing in May 2002, killing seven people.

The Rail Safety and Standards Board (RSSB) said since the type of points was introduced in the 1980s, their maintenance was "not fully understood".

But no "conclusive evidence of any one cause" of the crash was found, it said.

'Human issues'

RSSB chief executive Len Porter said: "It has taken a long time to thoroughly examine all the possible causes of this accident.

"Unfortunately the report does not provide us with conclusive evidence of any one cause.

[There] is a combination of mechanical and human issues plus a lack of proper maintenance procedure Len Porter RSSB chief executive

"What it does point to is a combination of mechanical and human issues plus a lack of proper maintenance procedure."

It remains unclear why the missing nuts were found loose on the ground, although private contractor Jarvis said soon after the crash that sabotage could not be ruled out.

However, the RSSB said on Tuesday that while malicious tampering with the points could not be ruled out, it was "highly unlikely".

A report by the Health and Safety Executive in May 2003 also found no evidence of sabotage or deliberate unauthorised interference with the points.

The standards board said it had not been able to establish with certainty how the missing nuts points had come to be in the condition in which they were found after the derailment.

Training lacking

The report questioned the common method used before the accident to position and secure nuts on the adjustable stretcher bars of the points.

It said the method "has been shown to contribute to their loosening and could have been a factor in this case".

Also, staff had not received specific training in relation to the points, it said.

The report made 29 recommendations to the industry, some of which have already been addressed.

Last year Network Rail and Jarvis accepted legal responsibility for claims brought over the Potters Bar rail crash, to provide "comfort and assistance" to victims.

Jarvis said the move did not mean the firm was to blame for the accident.

In response to the board's report, Jarvis highlighted that the problem of the adjustable stretcher bars was an industry-wide one.

"We hope that [the report] is another step towards providing the injured of the Potters Bar accident and the friends and relatives of the deceased with an explanation of what happened."

However, it added, that the board was still unable to give a "definitive explanation of why the faulty points which caused the derailment were in the condition they were at the time of the accident".