The rail operator confirmed all lines were blocked between the stations while emergency services worked to deal with the incident.
But the lines between Gatwick Airport and Haywards Heath have now reopened. Southern Rail said.
A spokesman said: “A reduced service continues to run between Gatwick Airport and Haywards Heath.
“Services are now able to call at Balcombe station again.
“If you are travelling this evening, we would advise to leave an extra 30-60 minutes for your journey.”
In a separate incident all lines were closed between Barnham and Horsham but they have now reopened.