Buses are replacing trains between Shrewsbury and Llandrindod Wells following extensive damage to the line caused by extreme rainfall during the recent storms.
Near Knighton, the railway has been damaged at various sites including across a 350m stretch where ballast has been washed away and significant debris has been washed onto the track meaning it is completely buried in some areas.
Network Rail engineers are working around the clock to inspect and repair the damage and will confirm how long the closure will last as soon as possible.
The closure means buses will replace trains between Shrewsbury and Llandrindod Wells with passengers advised to check before they travel at National Rail Enquiries or with Transport for Wales.
Bill Kelly, Network Rail’s Wales route director, said: “We are sorry for the disruption this has caused to passengers who use the Heart of Wales line.
“We saw extreme rainfall for a prolonged period which has caused significant damage to sections of the railway and left debris across hundreds of metres of track.
“We are already working around the clock to repair the damage as quickly as possible and we will keep everyone updated with our progress.”
James Price, Transport for Wales CEO said: “We would like to apologise to our customers for any disruption that has been caused, we’ll be working with our partners in Network Rail to resume services as quickly as possible.
“There will be rail replacement buses available and please could all our customers check online before they travel for updated information.”