Police officers dealt with an incident between Crowborough and Uckfield this evening, according to Southern Rail.
Southern said police attendance was required at Uckfield station, 'owing to a passenger causing a disturbance'.
It added: "While the police were dealing with issue, all lines were blocked between Crowborough and Uckfield.
"The passenger causing the disturbance has now been escorted off the premises, so trains are now allowed to run as normal between Crowborough and Uckfield.
"Trains between East Croydon and Uckfield are returning to normal. Trains may be delayed by up to 10 minutes or cancelled until approximately 8pm."