The signal that was blamed for the deaths of 31 people in the 1999 Paddington rail crash has been brought back into service.
The crash at Ladbroke Grove, near Paddington, killed 31 people
The inquiry into the crash concluded signal SN-109 was difficult to read in bright sunlight.
Network Rail said the signal had been repositioned and upgraded.
More than 400 people were also injured after a Thames Trains train went through a red light at Ladbroke Grove shortly after leaving Paddington.
The train operated by recently-qualified driver Michael Hodder hit a Cheltenham to Paddington Great Western express on 5 October 1999.
Lord Cullen's report into the crash said there had been a "lamentable failure" by maintenance firm Railtrack to respond to safety warnings on the stretch of track before the accident.
There was "a serious and persistent failure" by the company to look at how well signals could be sighted, the inquiry concluded.
Robin Gisby, director of operations at Network Rail, said it had taken on board all the recommendations from the enquiry.
He rejected criticisms from the train drivers unions, Aslef, that there had not been enough consultation about the decision, saying the signal had passed necessary safety checks.
"Obviously everybody's deeply sorry about what happened at the time, bringing it back now, as I say, will improve the operational layout of Paddington," he said.
"We've done everything we possibly can in terms of safety measures, to make sure such a tragic accident never happens again."
Chairman of the Paddington Survivors Group, Jonathan Duckworth, told BBC 5 Live that despite its central role in the crash, he was not opposed to the idea of the signal being brought back into use.
He said: "Initially your feelings are very strong, about the crash, about the fact that there was a signal that was difficult to see, that drivers didn't feel comfortable passing.
"But as long as all due process has been gone through, then I suppose our view is that the crash happened six years ago, and things have to move on from that."
Lord Cullen also said the culture of safety in regard to the training of drivers was "slack and less than adequate".
Thames Trains pleaded guilty to health and safety offences in 2004 and was fined £2m.
In November last year, the Crown Prosecution Service (CPS) said Network Rail, which took over Railtrack's duties after the firm went into administration, would face criminal charges under the Health and Safety at Work Act.