Trains have been cancelled between Carmarthen and Swansea due to a broken track.
Arriva Trains Wales is urging commuters to avoid "non-essential travel" following the fault between Ferryside and Kidwelly in Carmarthenshire.
Network Rail said the "extensive damage" to the track was caused by a freight train on Monday.
Trains are expected to be cancelled until Wednesday morning while repair works are carried out.
Replacement bus services are running between Carmarthen and Swansea via Pembrey and Burry Port and Llanelli in both directions until further notice.
Network Rail are advising passengers to allow an extra 60 minutes for journeys.