Commuters are facing delays of up to 30 minutes after the Brighton to London mainline became blocked.
Network Rail said the delay between Gatwick and East Croydon was due to an obstruction near Coulsdon, near Croydon.
It happened this morning and was still blocked at 8am.
Train operators said delays will continue until further notice.
First Capital Connect is accepting Southern passengers between London and Brighton.
For more details visit www.nationalrail.co.uk/service_disruptions/today.html, call 0871 200 49 50 or see Twitter hashtag: #Coulsdon.
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