Train operator First Great Western said on its website the hitch, which has affected services travelling between London Paddington and Reading, has been fixed but delays and some cancellations are likely to continue until 4pm today.
FGW tickets are being accepted on alternative routes from London Waterloo to Windsor and Eton Riverside as well as on Chiltern Railways and Cross Country services.
Visit www.nationalrail.co.uk for details.
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