Trains on the Uckfield Line were disrupted last night after the 4:33pm service from the town to London Bridge passed a red signal at Greenhurst Junction.
A spokesman for train operator Southern said it was a “low-speed” incident with “no danger to passengers”.
Greenhurst Junction is between Buxted and Crowborough.
The Southern spokesman added: “The cause of the incident is currently being investigated so we cannot give reasons why this happened.
“The incident meant that there were no trains between Crowborough and Uckfield for a time until the train involved was terminated at Crowborough and taken to the depot.
“Rail replacement buses were running between Uckfield and Crowborough, between East Grinstead and Uckfield and between Uckfield and Oxted.
“Southern tickets were accepted on Southeastern services between Tunbridge Wells and London.”
Normal service resumed at around 9pm.