There are delays of up to 30 minutes on CrossCountry and First Great Western services between Cheltenham Spa and Birmingham New Street. It follows an earlier signalling problem and it is not known when normal service will resume.
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The Post Office said it was 'business as usual' but unions said Saturday's industrial action was getting solid support.
Workers and managers are to stage another strike on Saturday amid a long-running dispute over jobs, pensions and closures.
Cloudy in the West with a few showers. Sunny in the East but cold.