All rail lines have been reopened after a person fell onto the track at Plumpton.
While the emergency services attended, the power was switched off and trains were unable to run between Haywards Heath and Lewes.
Southern is warning passengers to expect delays or disruption.
Disruption is expected until 2pm, according to Network Rail.
A spokesman said: “You can check your journey using the National Rail Enquiries real-time Journey Planner.”
Southern previously said: “Due to a trespasser on the railway all lines between Haywards Heath and Lewes are blocked.”