Thousands of commuters from Brighton, Hove and Portslade had a miserable commute to work this morning and face further delays and cancellations on the way home again.
Their trains were diverted or they faced a journey on a replacement bus after emergency repairs had to be carried out in the Balcombe tunnel.
Journey times were at least an hour longer for many.
Train services are suspended between Haywards Heath and Three Bridges and rail chiefs said that the tunnel would be closed for the rest of the day.
Network Rail said that they hoped the repairs to the tunnel roof would be completed by the end of the night, with a full service operating tomorrow.
A spokesman for Southern, the main train operator between Brighton, Hove and Portslade and the capital, said: “We are obviously running as many buses as we can.
“Of course it takes longer but we can’t run train through the tunnel until Network Rail says so.
“We’ve got about 30 buses running between Haywards Heath and Three Bridges.
“They are shuttling between the two and leaving about every five minutes (at the busiest times).
“We’ve got lots of staff on the ground trying to help people too.”
Despite running 30 replacement buses, passengers were having to queue for them at peak times.
The spokesman said that passengers were being offered information about alternative routes with other operators accepting diverted travellers’ tickets.
Some passengers travelled on the Littlehampton service changing at Barnham in the west – or took the Hastings service and travelled into Brighton along the east coastway.
First Capital Connect – the Thameslink operator – said that it was running rail replacement buses every ten minutes.
After the landslip just south of East Croydon in early August, Junior Transport Minister Norman Baker said that the situation underlined the need to restore the Lewes to Uckfield line.
He has campaigned to restore the link for years but, given his constituency interest, would not be allowed to make the decision.
Network Rail said: “A problem inside Balcombe tunnel is causing major disruption on the Brighton main line today.
“A number of beams which support the tunnel lining have become detached from the tunnel wall and, while the structural integrity of the tunnel is not affected, the tunnel has been closed so we can carry out a full inspection and make the necessary repairs.
“Train services are suspended between Three Bridges and Haywards Heath for the rest of the day, with rail replacement buses between those stations.
“Passengers are advised to check with National Rail inquiries or their train operator for the latest travel advice.
“Repairs in the tunnel are expected to be complete tonight and a full service should run tomorrow morning.
“We apologise for any disruption passengers have experienced today but it is right that we put safety first.”