Passengers are being warned of major disruption after heavy rain washed away parts of the Marches line between Abergavenny and Hereford.
Flash flooding on Saturday washed away the railway foundation stone leaving sections of track suspended.
The extent of the track damage caused by the extreme weather means the important route in and out of Wales will be closed for some time.
Network Rail engineers say the line is expected to stay closed until next Monday.
Rail replacement buses will be in operation between Newport and Hereford.
A Network Rail spokesperson said: “We understand how disruptive the closure of the Marches line will be to passengers and we’ll work as fast as we can to get it back up and running again."
"We thank passengers for their patience while we make our repairs and advise them to check National Rail Enquiries to find alternative routes while the Marches line remains closed.”
Passengers are being advised to check with their train operator or visit www.nationalrail.co.uk for the latest travel information.
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