Train passengers have faced delays and cancellations due to a signalling problem.
A spokesman for GWR said the problems were due to a signal systems fault in the the Slough area, and not a result of the new rail timetable launched on Monday.
The issue caused problems for commuters starting the new week, with routes into and out of London Paddington among those affected.
GWR said there were delays and possible cancellations between Reading and Paddington, affecting passengers travelling to London from the Gloucestershire and South Gloucestershire area.
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