Disruption on a section of rail line in Essex caused by a landslip is set to continue.
Damaged tracks between Billericay and Wickford, on the Southend Victoria to London Liverpool St line, were reported by a train driver on Wednesday.
Engineers found that ballast - the stones which help keep tracks safely in place - had fallen away under a section of track on the Southend-bound line
Greater Anglia said it expects delays until end of service on Friday.
Network Rail said its engineering teams were at the site to assess the damage and to try to establish how it occurred.
The Southend-bound line has been closed at the location of the damage "as a safety measure", a statement said.
Services are able to run in both directions on the adjacent line, but it means a reduction in the number of trains Greater Anglia can run.
Ellie Burrows from Network Rail said its "first priority is the safety of passengers and staff".
"We have already mobilised our supply chain to get the 500 tonnes of stone we need and will be focused on getting the embankment and tracks restored as quickly as we can," she said.
"We can run trains in both directions on this line, so while I'm sorry there will be some disruption for passengers, we will be working with Greater Anglia to run as many trains as we can for passengers."
Greater Anglia managing director, Jamie Burles, apologised to customers and said the operator was working with Network Rail "to restore the normal service as soon as is possible".
He said it was operating a reduced service "supplemented by buses between Wickford and Shenfield".
Passengers are asked to check before they travel and plan for slightly longer journeys.