Passengers on the London to Brighton line could have been killed because of years of maintenance work failures in a Victorian tunnel, a whistle-blowing rail engineer has claimed.
Balcombe Tunnel, near Crawley, West Sussex, was shut for 22 hours in September 2011 after workers discovered the partial collapse of a metal ceiling platform.
A report into the incident highlighted Network Rail failures that left large girders hanging inches above trains.
The Network Rail engineer, who does not want to be named, told BBC Inside Out South East if one of the steel beams had hit a carriage passengers would have died.
He said: "You're looking at a fatality if that steel bar would have come down.
"God forbid if it had gone down between two trains. Then it would have gone through both, ripped the train, [the] whole carriage, half a carriage - but there would have been fatalities."
'Problems not fixed'
More than 70 million passengers travel on the main London to Brighton line, with 2,500 trains passing through the Balcombe Tunnel each week.
The Rail Accident Investigation Branch (RAIB) report into the 2011 incident said engineers told the company in 2008 there were issues with the tunnel but "inadequate reporting" meant the problems were not fixed.
A total of 18 bolts were found to be missing and others had worked themselves loose from the metal platform, which was installed to catch water seeping into the tunnel.
The report also reveals the Network Rail engineer responsible for 120 tunnels including Balcombe was not suitably qualified or given the correct support.
The engineer added: "These tunnels should be inspected more than what they're doing now and with competent people, not someone, no disrespect, but not someone out of university or college with a degree in one hand and a torch in the other.
"They haven't got enough men on the ground as it is to do a track examination. What used to be examined three to four times a week is now examined in some parts once a week or once a fortnight.
"That's a little bit worrying."
James Abbott, editor of Modern Railways, said engineering staff had limited time to work on repairs because train routes were so busy.
He added: "There's a huge pressure in order to find the time to make the track available for the engineering staff to work on it because more and more people want to travel early in the morning [and] late at night.
"There's continual pressure on the window of opportunity - in the night time - to get on the track so the railway staff are always fighting that, that tension between running trains for passengers and finding the time to work on the track."
Tim Robinson, Network Rail's Sussex route director, admits its maintenance procedures were not good enough at the time.
He said: "Our processes let us, our colleagues and the travelling public down at that point.
"Communication wasn't appropriate and our processes weren't in place in a manner that allowed our staff to report failure of that nature, in the right place at the right time, to make sure they got recovered and repaired."
Network Rail said it had made changes to ensure a similar situation did not arise again.
Mr Robinson added: "From my perspective I can assure the general public and our customers that we have changed our processes. We have got much more local accountability.
"We're investing more money now than we were at that point and, from a Balcombe Tunnel perspective in particular, I'm now satisfied that we have an inspection regime and a re-engineered tunnel to make sure that risk has now been removed."
Inside Out South East is broadcast on Monday at 19:30 BST. It is also available nationwide for seven days thereafter on iPlayer.