Rush-hour commuters faced widespread travel disruption on Thursday after lightning struck an electrical substation, causing many trains to be cancelled.
Services in and out of London's Victoria station were the worst affected. The lightning strike at south Wandsworth, near the central London transport hub, caused a fire that badly damaged the substation, leading to a power failure and serious signalling problems.
A National Rail spokesman said the lightning had hit at 5.30pm - just as many people would have been leaving work to make the journey home. The problems are expected to continue until tomorrow.
The spokesman added: "We're not expecting to have it fixed tonight. It's a serious problem."
Rail company Southeastern bore the brunt of the signalling failure.
A spokeswoman said: "At the moment there are no services into or out of London Victoria. There are some services on other routes from London out towards Kent and Sussex, however.
"This is likely to cause problems on rail services in the South East for the whole of the evening."
Southeastern said passengers were being transported on alternative high speed routes, on London Underground or Southern railway trains or buses, until the problem was fixed.
Southern Railway was also affected, with services between Battersea Park and London Bridge suspended.
South West Trains told passengers there were delays of up to 40 minutes on services to and from London Waterloo because of the signalling problems.