Is London to Brighton's Balcombe rail tunnel safe?
30 September 2013 Last updated at 11:02 BST
Two years ago, part of the roof of the Balcombe tunnel on one of the busiest commuter routes in the UK collapsed.
The tunnel was immediately closed, but a subsequent Rail Accident Investigation Board report concluded passengers' lives had been put at risk.
Three metal girders had come loose and were found to be hanging less than a foot from the top of the train.
Mark Norman investigates the partial collapse on the London to Brighton line and speaks to a Network Rail engineer who claims passengers could have died.
Network Rail says it has re-engineered the tunnel to make sure the risk has been removed and it has instigated a more rigorous inspection regime.
Inside Out is broadcast on Monday, 30 September on BBC One South East and South at 19:30 BST as well as South East Today. The full film can also be seen nationwide for seven days after transmission on the iPlayer.