The incident occurred next to the Tonbridge-bound track in a ‘remote location’ near Crowhurst, west of Edenbridge, on Sunday December 22 and was caused by heavy rain.
Network Rail closed the line in both directions and two days later revealed it would need to build a temporary road, tunnel and bridge to address the issue.
It said: “The ground at the site of the landslip is still moving and too dangerous to work on. The location of the landslip is very remote, which means access is difficult.
The infrastructure company added: “After a month’s worth of rain in one week the soil had become saturated and nearby the River Eden had burst its banks.
“Kent endured three months’ worth of rainfall in November alone. The land around the line could not soak up any more rain.”
The line is one of Britain’s earliest railways, having been built on a raised earthen embankment in the late 1830s.
The Victorian embankment subsided, leaving the tracks hanging unsupported. Engineers believe the collapse may have been caused by rotational failure, where the surrounding land rises up while the railway sinks.
On Friday [December 27] Network Rail revealed that movement was ongoing and putting the other track at risk, too.
A replacement bus service is being operated by Southern, or alternative routes go via London Bridge and East Croydon.