Southern said there is major disruption to train services between Haywards Heath and Brighton.
The company said trains may be cancelled, delayed or diverted via Lewes.
All lines reopened by 7.50am this morning but Southern said disruption is expected until 11am.
The company is advising passengers to travel later, wherever possible, or use an alternative route.
Services on all routes via Haywards Heath are expected to be severely disrupted.