A fault with the signalling system at Clapham Junction hit Southern and Gatwick Express trains on Monday, causing headaches for commuters and holidaymakers.
There were no Gatwick Express trains running in the area, with National Rail working to reintroduce services on Monday afternoon.
Lines have reopened but there may still be changes to services and disruption is expected until 2pm.
London Overground and South Western Railway services were not being affected by the problem at Clapham Junction.
