Engineering work and signal problems are causing train delays and cancellations between Reading and Slough.
First Great Western say that emergency engineering works near to Twyford are causing delays of about 20 minutes from all stations.
Trains have also been cancelled at short notice.
The rail company was unable to say when the problem will be sorted.
First Great Western said: "An estimate for the resumption of normal services will be provided as soon as the problem has been fully assessed."
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