Trains were stopped between two Kent stations due to reports of a trespasser.
No services ran between Dartford and Rochester while emergency services dealt with the incident.
Southeastern announced at just before 4.20pm that power had been turned off and the line was being inspected.
Emergency services arrived at the scene and were “dealing with an incident”.
All lines have since reopened but Southeastern says service recovery is ongoing.
The rail operator says trains may be delayed and revised.
A statement on X said: “The emergency services have completed their work following an earlier incident between Dartford and Rochester.
“Trains running between these stations may still be cancelled, delayed by up to 60 minutes or revised whilst service recovers. Disruption expected to 19.00.”
Southeastern announced commuters may use tickets at no extra cost on Arriva 190, Fastrack buses and Thameslink trains via reasonable routes.
A Kent Fire and Rescue Service service spokesperson said crews were called at 4.24pm to Canal Road, Strood to assist Kent Police, British Transport Police, SECAmb and Southeastern. One fire engine attended.
