The reliability of the Automatic Train Protection system fitted at the lineside and to high-speed trains was "open to question", a company spokesperson said.
The system was still being tested as a supplementary safety mechanism to existing signalling on the main line from Paddington to the west, Railtrack disclosed. Those tests had shown that where the equipment is an "add- on", then it could prove unreliable.
The company said that it did not know whether the ATP installed by the trackside, sending signals to a computer in the cab of the high-speed train, was switched on.
However, a senior official of the footplatemen's union, Aslef, to which the driver of the doomed Swansea-Paddington express belonged, insisted: "We knew the ATP was not turned on because it was in such a state of disrepair.
"As a result, the driver was robbed of an essential safeguard against human or any other error that might have taken place."
The driver, an experienced footplateman working from Old Oak Common depot in west London, was released on police bail early yesterday after answering questions related to possible manslaughter charges.
Other rail experts said the ATP system, which should prevent trains going through a red light, had only been installed on an experimental basis on the route and the trials had now ended. Its status will be a major question for the forthcoming inquiry.
Mel Holley, deputy editor of Rail magazine, said: "I thought the ATP trials were over. They were certainly nearing the end of the trial. They did stretch from Reading to Paddington. It was just to prove whether the technology worked, and to see how much it cost. The last government said it was too expensive per life to be installed." Great Western has a fleet of about 100 High Speed Trains. "Not many of them had APT fitted," said Mr Holley.
Installation of the fail-safe system throughout the UK rail network was recommended by the Hidden Report into the Clapham train crash of 1988, in which 35 people died. The then Transport Secretary, Cecil Parkinson, promised that no expense would be spared to make the railways safe, but vetoed ATP on the grounds that it would cost pounds 600m.
Last night, Railtrack began clearing the main line from Paddington to the West Country and Wales, but services are not expected to resume until tomorrow. In the meantime, travellers are being diverted to South-West Trains services via Waterloo.
After the first major accident since rail privatisation, establishing liability for the accident is likely to prove a headache.
Mr Holley thought that signal failure could not be discounted, but it was unlikely to be the cause, since such problems would tend to have emerged earlier. "If you consider Clapham, that happened on a Monday morning with one of the first trains. Here, you have a busy section of freight track and a high-speed passenger train in the middle of the day."
Lawrie Harris, a spokesman for the RMT rail union, said: "As yet there is no proof, but the indications are that it is a Spad - Signal Passed At Danger - incident. British Rail recognised this was a problem in 1986. That is why they introduced ATP, but it was effectively dropped in 1994 for cash reasons. The government was privatising the railways and thought no one would want to pay for pounds 600m worth of safety equipment - despite Parkinson saying after Clapham that money would not be a problem."Reuse content