The crash claimed 31 lives

But the BBC has learned that the companies could still be prosecuted - under the Health and Safety at Work Act 1974 - over the crash which killed 31 people died.

Although prosecutions under health and safety law are regarded as lesser charges than manslaughter or gross negligence, the penalty available to the courts is the same - an unlimited fine.

Following the 1997 Southall rail crash, in which seven people died, Great Western was fined �1.5m after being prosecuted under the Act.

'No retribution'

More than 400 were injured in the crash near Paddington, west London, on 5 October 1999.

The Crown Prosecution Service has advised the British Transport Police that further investigations would not be justified.

Victims' families are still pressing for safer railways

Martin Mimms, a spokesman for the Paddington Survivors Group, said : "I think survivors were prepared for no charges to be made. They are not interested in retribution."

The group's founder Pam Warren, who was badly burned in the crash, said: "What we want is a safer rail system and greater corporate responsibility."

But solicitor Louise Christian, who represents most of the bereaved, said: "My clients are devastated by this decision and we will attempt to challenge the decision in the High Court on a point of law."

The collision happened when a Thames commuter train leaving Paddington crashed almost head-on with a Great Western express from Cheltenham, after passing through a red signal.

The CPS investigation had focused on the training of Thames train driver Michael Hodder.

Training 'deficiencies'

It had also probed the actions of signal box staff, and the track and signal layout in the area.

The CPS said there were a number of "deficiencies" in the training which Mr Hodder received.

But it said it would be impossible to prove that he passed the signal because of negligent training.

Nor could it be proved that a director or senior manager of a company was personally responsible for the actual events that led to deaths.

The CPS said it could not prove gross negligence by the signalman on duty at Slough, even though he was slow to signal danger to the Great Western train, and no attempt was made to divert the Thames train.

"In essence the fault of the signalman may be expressed as just seven seconds' unexplained inactivity. We do not think that we could prove that this inactivity amounted to gross negligence."

Lessons

A spokesman for the train drivers' union, Aslef, said: "Lord Cullen, in his second report into this tragedy, found that both Railtrack and Thames Trains bore a share of the blame.

"This corporate responsibility by the privatised industry should not be forgotten in the light of today's announcement."

Railtrack said it was continuing "learning the lessons" of the inquiry.

And Thames Trains said: "Safety has always been our number one priority...our main focus now is actually working through all the recommendations from the Cullen report and implementing them as quickly as possible."

The British Transport Police is expected to formally announce a decision on whether to drop its investigation in a few days, but the CPS advice effectively ends the matter.