Network Rail, which inherited the liabilities of Railtrack, was convicted of breaking safety rules before the crash.

Four people died and 102 were injured when a rail shattered under a King's Cross to Leeds train travelling round a curve at 115mph on October 17, 2000. The rail had been identified as faulty 21 months earlier but had been left unrepaired.

Railtrack failed to impose a speed restriction on the line despite several reports warning that the rail was deteriorating. When the deadline was missed for repairing 200 track defects, Railtrack simply allowed a new deadline to be set.

The prosecution had alleged that the derailment occurred because of a cavalier approach to safety by Railtrack, its contractor Balfour Beatty and five managers working for the two companies. The defence argued that it was unfair to make the managers scapegoats.

Jonathan Goldberg, QC, defending one of the men, told the court that they worked in an underfunded industry that had been neglected by successive governments for more than 40 years.

Balfour Beatty and the five men had originally faced manslaughter charges but these were thrown out by the judge, Mr Justice Mackay, halfway through the trial. Charges of manslaughter against Railtrack and three executives, including Gerald Corbett, the former chief executive, were dropped before the trial began. Balfour Beatty admitted breaching safety rules after being cleared of corporate manslaughter.

The case again highlighted the difficulty of securing convictions of companies for corporate manslaughter under current legislation.

The Government has proposed reforming the law to make it easier to prosecute companies for manslaughter by removing the need to prove that a senior individual was responsible. The draft Bill is due to be scrutinised by Parliament in the autumn.

John Pickering, the lawyer for the victims' families, said some of them felt betrayed. "They feel rather let down by the prosecution process and the fact that there haven't been successful prosecutions other than on basic health and safety charges against two companies," he said. "The law as it currently stands is unsatisfactory and proves ineffective in establishing responsibility at a senior level."

Mr Pickering said that Labour had been dragging its feet since promising to reform the corporate manslaughter law in 1997. "I am hugely sceptical about the speed with which this process is moving."

Mr Justice Mackay, in dismissing the manslaughter charges last month, said: "This case continues to underline a long and pressing need for the long-delayed reform of the law in this area of unlawful killing."

Bob Crow, the general secretary of the Rail, Maritime and Transport union, said: "Now more than ever before we need an effective corporate manslaughter law to ensure that bosses responsible for avoidable deaths are held to account."

The five managers acquitted yesterday were Anthony Walker, 48, Balfour Beatty Rail Maintenance Limited's regional director, and Nicholas Jeffries, 50, a civil engineer for the same firm; Alistair Cook, 52, and Sean Fugill, 52, both managers for Railtrack North Easter, and Keith Lea, 55, a Railtrack LNE track engineer.

Mr Goldberg told the jury: "These five men worked in an under-funded, under-invested railway industry, which had been neglected by governments of all parties for over 40 years and which had recently undergone a botched and unworkable privatisation.

"It is a sad reflection on political correctness and the blame culture of modern-day Britain that five men at modest job levels are blamed for Hatfield while the concerned and grieving relatives and the press and public are fed the line that the buck stops with them."

The four men who died in the crash were Steve Arthur, 46, from Pease Pottage, West Sussex; Peter Monkhouse, 50, of Headingley, Leeds; Leslie Gray, 43, of Tuxford, Nottingham; and Robert James Alcorn, 37, of Auckland, New Zealand.

Network Rail and Balfour Beatty will be sentenced on October 3 for the health and safety offences.

Ian McAllister, the Network Rail chairman, apologised in a statement to the victims and said that the company, which replaced Railtrack in 2002, took all maintenance back in-house last year.

CASES DISMISSED

KEITH LEA, 55, was track engineer of the London North East Zone of Railtrack and oversaw work carried out by maintenance contractors. Having spent more than 35 years in the industry, he is still regional track maintenance engineer for Network Rail, ensuring that correct track standards are met by maintenance teams.

SEAN FUGILL, 51, at the time of the crash was the area asset manager of Railtrack's London North East Zone. He has spent 26 years working on the railways and is now transition manager for Network Rail and is part of a team working on the transfer of maintenance back to the company for the Reading area.

ALISTAIR COOK was the asset manager for the London North East Zone of Railtrack from King's Cross to the Scottish border on the East Coast mainline. Mr Cook, 52, oversaw track and signals maintenance, and reported directly to Nicholas Pollard. He is now head of maintenance improvement at Network Rail's headquarters.

NICHOLAS JEFFRIES, 53, was employed by Balfour Beatty and was responsible for overseeing all engineering work and for advising his line management on engineering matters.

He left his £50,000-a-year job with Balfour after the incident but still works in the industry.

ANTHONY WALKER was working with Balfour Beatty as its regional director in charge of the contract to maintain the East Coast main line.

Mr Walker, 48, had worked on the railways for several years before joining the comany in 1996. He left his £50,000-a-year job before to the crash.