Rail engineers have begun working on stretch of track blocked by a landslip since Christmas Day.
A landslip at Ockley, caused by heavy rain during December, means trains can't run between Horsham and Dorking.
Network Rail has apologised to passengers and says it hopes to have the area reopened by Monday (4 January).
The line through Ockley runs on an embankment built during the Victorian era, constructed using local Wealden clay.
Derek Butcher, a geotechnics engineer for Network Rail, said it was "not a material we would build railways on these days."
"Clay absorbs water like a sponge and once it reaches saturation – where it can’t absorb any more - it loses its strength," he explained.
"In this case part of it has slid out from underneath the weight of the railway embankment."
The slip happened at some point between Christmas Eve and Boxing Day, with the London bound line worst affected - but Network Rail says both tracks must shut for safety reasons.
Shaun Kind, regional director for Network Rail, said they hoped to "return the railway to full use on 4 January, at the start of the working week, however that depends on a lot of variables we are still working on, including ground surveys and ecological checks."
Andre Harlock, Train Services Director at Southern, said: "I'm sorry for the disruption the landslip is causing. There are far fewer people travelling due to the Christmas period and pandemic restrictions, but those who are will need to take a different route, so please allow extra time."
Passengers are being urged to check online before they travel.