Police investigating the Hatfield rail tragedy are understood to be considering charges of corporate manslaughter against firms responsible for maintenance of the track.

Transport police detectives are believed to have visited Railtrack's regional headquarters in York and the offices of construction giant Balfour Beatty in Hatfield and are understood to have taken away a number of files.

A spokesman for the British Transport Police is understood to have told a national Sunday newspaper: "We are examining the evidence and the charge of corporate manslaughter could well be brought."

Regional Railtrack boss Nicholas Pollard said today his company would co-operate fully with any investigation into the tragedy. He said the police investigation was just one of several inquiries under way. "We are fully co-operating with all aspects of every inquiry," he said.

Four people died and dozens were injured when the GNER London to Leeds express derailed last month.

A cracked section of rail is believed to be partly responsible.

Mr Pollard has already admitted in an exclusive interview with the Evening Press that his company knew there were problems with the rail - but not how bad the condition was.

Railtrack has overall responsibility for rail maintenance, but actual inspection and maintenance work for the East Coast main line is contracted out to Balfour Beatty.

If charges of corporate manslaughter were brought, it is possible that individual managers at the rail firms involved could be held personally responsible.