Damaged overhead wires are causing major disruption between London Paddington, Heathrow Terminals and Reading.
The damage has occurred between Ealing Broadway and Southall.
Great Western Railway, Heathrow Express and Elizabeth Line services are expected to be affected until this afternoon.
Services may run late, be revised or cancelled at short notice, so customers are heavily advised to check before travelling.
Some ticket acceptance will be in place for those travelling on the Elizabeth line. Great Western and Heathrow Express customers should be aware that a reduced service is running.
Journeys can be checked online.
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