Britain's railway lines have been heavily criticised by experts in a BBC documentary being shown this week.
Network Rail insists the track on Forth Rail Bridge was safe
The BBC 1 Whistleblower programme claims parts of the UK's railways are "dangerous" and of "great concern".
The track on the Forth Rail Bridge was singled out for concern and a 20-yard stretch elsewhere was held in place by only one safety clip.
Dozens of rail insiders helped construct the programme, set up by three reporters who worked undercover.
The probe also found a track fault was wiped from computer records a week after a derailment occurred.
The makers of the programme also revealed that an examiner allowed a candidate to cheat in a track exam.
Track cards, vital to workers' safety, were often not checked for days on end and were in clear breach of safety rules, according to the programme.
Professor Brian Clementson, former director at British Rail and Virgin Trains, said: "I found the whole thing very worrying and very concerning. And I hope that Network Rail will take this film as being a positive contribution to trying to improve things."
But the BBC approached Network Rail, which provided a
written statement: "The track on the Forth Bridge is safe and in a satisfactory condition.
"The reports we have of the track quality and condition on the bridge taken over the last year can establish that it is fit for purpose.
"The conditions you describe are not a safety risk and do not require immediate attention or speed restrictions."
The firm added: "We wish to state that the safety of the
railway and its operations is without question our paramount priority.
"The company has taken the allegations extremely seriously and has completed as full an investigation as possible.
"This investigation has concluded that passenger or train safety has not been compromised."
The programme will be screened on Thursday.