There is severe disruption on train services between Reading and London Paddington. Passengers are being advised not to travel unless their journeys are essential.
The cancelled services are being blamed on overrunning engineering works and an electrical fault.
Stations affected include Newbury, Swindon, Oxford, Reading Twyford and Maidenhead. There is a limited road replacement service running, with 15 buses.
First Great Western customers can use their tickets on South West Trains services between Reading and London Waterloo.
Services have also been suspended on Heathrow Connect between London Paddington and Heathrow Airport.
Network Rail has apologised for the disruption and said it was working to reopen all lines as soon as possible.