The incident happened in the Wivelsfield area this morning.
The lines have been re-opened but Southern services between Haywards Heath and Brighton remain severely disrupted.
Where available Thameslink and Gatwick Express will accept valid Southern tickets over any reasonable route, however these services are also severely disrupted as a result of this problem.
Southeastern Trains are accepting tickets between Hastings and London.
South West Trains are accepting your tickets between Southampton and Portsmouth and London.
A limited replacement bus service is in operation between Brighton and Haywards Heath and rail tickets are also being accepted on bus services operated by:
Stagecoach (Route 700) between Brighton and Chichester,
Metrobus (Routes 270, 271 and 273) between Brighton and Haywards Heath,
Brighton and Hove (Route 2) between Brighton and Shoreham.