The incident, which occurred on Monday (27 January) between Three Bridges and Gatwick airport, has blocked some of the northbound lines.
Gatwick Express services are suspended in both directions, although Southern Rail and Thameslink services – which also serve Gatwick – are continuing to operate.
The operator is warning passengers “it may take at least 10 minutes longer than usual" to reach their destinations, and has advised passengers to allow an extra 30 minutes to complete their journeys.
