Engineers are working to assess the risk from a landslip, which has caused disruption to rail users in the Haywards Heath area.
An automatic sensor alerted Network Rail to the collapse on Sunday afternoon, and the track was closed while initial inspections were carried out.
The landslip did not appear to encroach on the railway line, but Network Rail Kent and Sussex said a first responder made the decision to move all trains to one of two tracks.
“We will be watching the slip to make sure it’s still safe to run and over the next 24 hours we will get specialists to site to plan how we fix the problem and what sort of impact it will have on passengers,” a spokesman said.
Later, the spokesman added: “We're not going to be able to fix it, but we can hope to shore it up.
“Our team will scoop up as much of the debris as we can, and then crane one-ton bags of stone in place as a temporary wall.”
Speed restrictions were put into place, and all services through the Haywards Heath area experienced delays.
During the closure, passengers were able were able to use tickets on local Metrobus services
This morning, Network Rail confirmed the line had re-opened but with speed limits in place. They thanked passengers for their patience and urged travellers to check timetables before travelling.