The cross-border rail link between Belfast and Dublin could remain closed for a number of days because of difficulties in locating the source of explosions reported on the line near Newry yesterday.
A British army helicopter and spotter plane are conducting a search but have so far been unable to find the location of the blasts.
The RUC says it has identified a number of sites which would require further examination but its officers would have to be cautious in the search because of the terrain and what was termed "the current terrorist threat".
The line between Portadown and Dundalk will remain closed and Translink say buses will be laid on to transport passengers between the two stations.
