Balfour Beatty, which is still involved in major railway projects, was responsible for a track defect which put more than three-quarters of a million passengers at risk when they passed over the line near the Hertfordshire town, Mr Justice Mackay said.
The judge also fined Network Rail £3.5m yesterday for breaching health and safety legislation. Both companies were ordered to pay £300,000 each in costs in a case that incurred expenditure estimated at £20m. He said the engineering firm's culpability was "two or three times" as much as Network Rail formerly Railtrack. Network Rail was convicted of breaching the Health and Safety Act last month and Balfour Beatty had admitted the charge earlier.
Four people died and 102 were injured when the King's Cross to Leeds train came off the tracks at 115mph on 17 October 2000.
The prosecution maintained the derailment, caused when the track " disintegrated", was an accident waiting to happen and occurred because of a cavalier approach to safety. The judge said both companies fell below appropriate standards, but Balfour Beatty's failure lay at the "top of the scale". The judge who has spent 30 years involved in similar cases said he had guarded against over-reaction in sentencing. " But I regard Balfour Beatty as one of the worst examples of industrial negligence in a high-risk industry I have seen," he said.
Balfour Beatty is no longer in the business of railway maintenance because Network Rail has taken over from Railtrack and taken maintenance "in house", but it is still involved in major projects on the system.
The judge said: "No one can predict the future, but the risks of such a tragedy had been reduced by the action of Network Rail. The elimination of one of the indefensible features of the 1996 privatisation the separation of the ownership and control of the track from its maintenance is now gone. Perhaps that is one good thing resulting from this disastrous affair."
A corporate manslaughter charge against Balfour Beatty responsible for track maintenance was thrown out by Mr Justice Mackay. His decision highlighted the difficulty of securing convictions against companies for corporate manslaughter under present legislation. Fresh calls have been made for an urgent change in the law.
After it was cleared of corporate manslaughter, Balfour Beatty admitted the company had broken safety rules. Five rail executives were also cleared earlier this year of both manslaughter on the direction of the judge and of health and safety breaches, by the jury.
The court had heard there was a total vacuum in management and supervision of track inspection on the busy line near Hatfield.
The fines were welcomed by the Safe Trains Action Group (Stag). Its vice- chairman, Carol Bell, said: "We have said that there have to be bigger, swingeing fines for companies and it's good to see that there have been in this case."
Balfour Beatty said that in entering its guilty plea, it had accepted inadequacies and apologised. "It is, however, clear that the accident arose as a result of a systemic failure of the industry as a whole. At no stage did BBRIS [the Balfour Beatty subsidiary] work outside industry standards on patrolling and inspection as they were constituted before the accident."
Audrey Arthur, whose son Steve was killed in the crash, said after the fines were announced: "I don't think justice was done." Speaking on Sky News, she said: "I think of my son all the time and I can take no satisfaction from the fines.
"Within a month they will have forgotten about all this. But we have got it with us for the rest of our lives."