Commuters between Oxford and London are facing delays and cancellations today.
Trains running between London Paddington and Reading are expected to be disrupted until the end of the day after a broken rail was found in west London.
National Rail alerted customers to the problem this morning.
Affected lines will include Great Western Railway, the Elizabeth Line and the Heathrow Express.
A spokesperson from National Rail said: “A broken rail between London Paddington and Slough/Heathrow Terminals means that fewer trains are able to run on some lines.
“During routine checks, Network Rail engineers have found a defect with a rail on the high speed line towards London Paddington.
"Due to this, the defect will be monitored whilst trains run at a reduced speed.
“Due to the high frequency of services in this area, and the delays this will impose on them, the decision has been made to amended some services to minimise the level of disruption to passengers.”
Trains may be cancelled, delayed by up to 15 minutes or revised.
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