Train services between Oxted in Surrey and Uckfield in East Sussex are running again following repairs to a track defect, albeit at a reduced speed.
Southern Rail had anticipated the line not reopening until 08:00 BST on Tuesday, but said engineers had now repaired the defect and trains were travelling between the two stations.
The rail operator advised passengers to expect delays of at least 20 minutes and said the speed restrictions were expected until 12:00 BST.
The disruption comes after a train derailment in Lewes caused ongoing delays between Lewes and Haywards Heath, while the track between Eastbourne and Polegate is open again with a revised timetable following a landslip on Saturday.
Southern said: "Engineers have now repaired a track defect near Edenbridge Town. Trains can now run on this route, albeit at a lower speed than normal."