First Great Western says services running through these stations may be delayed by up to 20 minutes at short notice.
It added an estimate for the resumption of normal services will be provided as soon as it has fully assessed the problem.
Check the First Great Western site for details.
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The Met Office has issued a yellow alert for Windsor and Maidenhead, Slough and Wokingham, warning that rain moving eastwards across the south of the country may turn to snow in some places.