Heathrow Express, the main line linking London to the airport, has been closed by fire as thousands of passengers attempt to board flights for the Christmas holiday.
The branch route to Heathrow was shut after water penetrated a line-side transformer at its junction with the Great Western main line earlier, causing a fire.
A second fire damaged equipment at Acton, closer to Paddington station in London.
Network Rail engineers, who worked through the night, said the routes should open again later today.
London Heathrow is expected to attract 105,000 departing passengers today, the most during the Christmas period.