Trains between Bristol, Swindon, Reading and London Paddington are 'severely disrupted' due to major signalling problems around Twyford.
Some lines through Twyford have now reopened but First Great Western (FGW) is warning travellers to still expect cancellations and delays.
FGW tickets are currently being accepted on some Chiltern Railways, Southern Railway, CrossCountry Trains and Transport for London services.
Follow @FGW on Twitter for updates.
Top Ten Articles
It's time to get voting as readers get their chance to decide the winner of the Independent Retailer of the Year at the inaugural Windsor and Maidenhead Business Awards.