Buses will replace trains between Clarbeston Road and Fishguard Harbour from Saturday, January 25 until Sunday, February 9 inclusive.
Buses will also replace trains between Carmarthen and Pembroke Dock from Saturday, February 1 to Friday, February 14 inclusive.
Between Clarbeston Road and Fishguard Harbour, Network Rail engineers will be carrying out a programme of safety-critical vegetation management. Teams will be removing potentially hazardous trees and vegetation, including diseased, dead, or dying trees.
At Spittal Tunnel (near Rosedown cemetery), the road will be closed from Monday, January 27 to Friday, January 31, between 9.30am-3.30pm each day, to allow teams to safely remove trees directly above the bridge. Engineers will also be renewing the track at Trellewelyn near Fishguard.
A replacement bus service will run between Fishguard Harbour and Haverfordwest via Fishguard & Goodwick, and between Fishguard Harbour and Carmarthen calling at Clarbeston Road, Clunderwen, and Whitland.
Between Carmarthen and Pembroke Dock, engineers will be replacing more than 1,500 yards of track at two locations – Narberth and Pembroke.
At Narberth Tunnel, they will replace 977 yards of track and 600 yards of drainage, replacing 1,400 sleepers and 3,900 tonnes of ballast (track stone).
At Pembroke Tunnel, they will replace 582 yards of track and 600 yards of drainage, replacing 815 sleepers and 2,500 tonnes of ballast.
Replacement buses will run between Carmarthen and Pembroke Dock. A mixture of buses will run, with some calling at all stations while others will make limited stops (Whitland and Tenby only).
Station Road, Manorbier, will be closed from Monday, February 3 to Friday, February 7, and again from Monday, February 10 to Friday, February 14, between 7am and 4pm.
Nick Millington, Network Rail Wales and Borders route director, said: “During Storm Bert and Storm Darragh at the end of last year, we recovered vast amounts of fallen trees, branches, and other items on the track. Fortunately, there were no injuries to passengers or train drivers, however, there is still more to do.
“Our number one priority is the safety of the public, passengers and railway staff, and our work between Clarbeston Road and Fishguard Harbour will help to make the railway safer.
“We are also working really hard to improve train service punctuality and the essential track renewals in Pembrokeshire are a part of our commitment to do that.
“We recognise there is never a good time to close the railway, but we have planned the renewals to minimise disruption as much as possible. I would like to thank passengers for their patience and urge them to check their journeys before travelling.”
Passengers are advised to check before they travel via the Transport for Wales journey planner at TfW's journey planner or National Rail Enquiries