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.
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The crash, which took place at about 7am today (Thursday), involved three vehicles. One man sadly died as a result of the collision and his next of kin have been informed.