Trains are currently unable to run between London Paddington and Slough or Heathrow Airport after 500 metres of overhead electric wires suffered "extensive damage" at Ealing on Tuesday night.
Many trains became stranded between Paddington and Reading due to a loss of power supply, and Great Western Railway said a reduced timetable would impact most mainline routes.
A spokeswoman said,
Travellers trying to get to Heathrow were told to take the Piccadilly Tube line or a bus-replacement service.
Further delays on the Central, Circle, District, Hammersmith and City, Jubilee and Metropolitan London Underground lines made it diffcult for commuters trying to get across the city.
GWR staff wheeled a trolley on to the Paddington concourse at around 8.30am to give away free tea, coffee and biscuits to frustrated commuters.
One customer tweeted,
Network Rail said it expected to open two train lines by midday, but warned disruption would last until the end of Wednesday.