Train services between Didcot Parkway and Swindon are suffering delays of up to 30 minutes due to signalling problems.
Network Rail engineers were called to deal with a fault at Bourton, near Shrivenham.
It is the second day that signal problems on the route have affected First Great Western services. Yesterday evening a fault at Challow, near Wantage, also caused delays.
For the latest information, see the FGW website.
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