The incident, which occurred next to the Tonbridge-bound track in a ‘remote location’ near Crowhurst, west of Edenbridge, on December 22, was caused by a month’s worth of rain in one week.
The track has been closed in both directions between Edenbridge and Godstone since then.
Commuters from Tonbridge are having to use a shuttle train to Edenbridge and then a bus to Redhill.
The infrastructure company said it would need to build a temporary road, tunnel and bridge to address the issue.
But repairs were hampered because the ground was still moving and was ‘too dangerous to work on’.
Recycled railway ballast has to be brought to the site by rail. It will build up a shallower slope on the 12-metre high embankment.
In total, 40,000 tonnes of the rock will be needed, with each train carrying around 1,000 tonnes – some of them coming from the northernmost parts of the country.
Plans are also underway to reinforce a longer 400-metre section in the area, but will not affect train services.
Network Rail’s Route Director for Kent, Fiona Taylor, said: “This landslip is a serious challenge for us, not just because of the scale of the slide but also the remoteness of the location.
“To give you some idea of the size of the slip, we are having to bring 40,000 tonnes of stone in from as far away as Carlisle to build the bank back up again.
“We’ve had to cut through the remaining part of the railway just to gain access to it.”
She added: “Our passengers have been very patient with us and we are doing everything we can to speed this project along and working 24/7 to get the railway open.
“Once we have an exact date we will let everyone know and if we can achieve that earlier than the end of March then we will.”
Southern’s Customer Services Director Chris Fowler said: ““We really appreciate our passengers’ patience while this section of railway is rebuilt.
“Everyone is working together to keep passengers moving with replacement services and alternative travel options, including ticket acceptance on other lines and at other car parks.”
