First Great Western services between Didcot Parkway and Swindon are being disrupted by flooding at Challow.
Cancellations and delays of up to 35 minutes can be expected.
FGW services are also affected by flooding at Wooton Bassett, west of Swindon.
Chiltern Railways services through Haddenham & Thame Parkway have resumed, after water blocked the tracks at the station earlier, but are still subject to severe delays.
Chiltern tickets are being accepted on FGW and CrossCountry Trains services as a result of the disruption.
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