Railway lines have re-opened following an incident attended by emergency services.
But Southern Rail said trains running between East Croydon and Clapham Junction may still be cancelled or delayed by up to 60 minutes.
Disruption is still expected until 10pm.
At least one train from Brighton to London Victoria was cancelled.
A Southern Rail spokesman said: "Lines are now open, but services remain heavily disrupted. Some trains may still be at a stand - we're working to get these moving again as soon as possible.
"Delays, alterations and cancellations are all possible, and some services may be diverted to alternative destinations, for instance London Bridge, or may terminate at a different station to normal.
"Please travel later today if you can. If you travel now, you will likely need an alternative route to complete your journey. Journey times will be extended by at least 30-60 minutes so please allow plenty of extra time."