Travellers are being warned they face severe disruption heading in and out of London for much of Wednesday.
Trains are unable to run between London Paddington and Slough or Heathrow Airport after overhead electric wires were "severely" damaged at Ealing on Tuesday night.
Many trains became stranded between Paddington and Reading due to a loss of power supply.
Network Rail said it expected to open two of the four train lines by midday, but warned disruption would last until the end of the day.
Delays and cancellations
Southern, Southeastern, Thameslink and Gatwick Express have all reported cancellations and delays of up to 45 minutes.
Great Western Railway (GWR) added that a reduced timetable would impact most mainline routes to and from Paddington.
The operator said: "Owing to the extent of the damage, no trains are expected to be able to operate between London Paddington and Hayes & Harlington in either direction for much of Wednesday.
"As a result, GWR, TfL Rail and Heathrow Express will not be operating a train service between London Paddington, Hayes & Harlington or Slough until such time as repairs have been completed."
It added: "If you are travelling from London beyond Reading please do not go to London Paddington but instead travel straight to either London Waterloo, London Marylebone or London Euston depending upon your ultimate destination."