Network Rail says Storm Ciara, which bought 90mph winds, extreme rainfall, and flooding across Wales, has caused extensive damage on the Conwy Valley Line, north of Llanrwst.
Engineers have identified several sites where the ballast supporting the track has been washed away by floodwater. The storm has also damaged fencing and level crossings at several locations.
Further assessments will be carried out once the floodwater has receded, but early indications suggest that the damage is not as extensive as that caused by Storm Gareth in March last year, when the line was closed for four months.
While the line remains closed, Transport for Wales will operate a rail replacement bus service to keep passengers moving but passengers are urged to check before they travel.