A railway line will remain closed for a week after flooding caused parts of the track to be washed away at the weekend.
Network Rail said there was "significant damage" between Abergavenny, Monmouthshire and Hereford, Herefordshire.
It said engineers were working around the clock at six locations and the line would be closed until 4 November.
Rail replacement buses are in operation as 500 tonnes of ballast and 300 tonnes of foundations are replaced.
An amended timetable is in operation between Newport and Abergavenny, with some rail services running alongside a rail replacement bus service.
The Met Office said more than 4in (100mm) of rain fell in 24 hours in some places.
Chris Pearce, Network Rail's head of operations delivery, said: "Significant damage was caused to the railway line and our engineers are working day and night to carry out these essential repairs and reopen the line as soon as possible.
"We would like to thank passengers for their patience and to also thank our staff and partners at Transport for Wales who worked tirelessly over the weekend in very difficult conditions to keep passengers moving as efficiently and as safely as possible."
Transport for Wales advised passengers to check before travelling.