The cross-border rail link between Belfast and Dublin remained closed tonight following the failure by the British Army after a daylong search to find the sites of at least two explosions.
A British army helicopter and spotter plane failed to find the location of blasts heard in the vicinity of the line outside Newry, County Down.
The RUC said it had 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 was termed"the current terrorist threat".
The suspension on normal services caused difficulties for 2,500 rugby fans who were booked on regular services and special trains from Belfast to Dublin for the Ireland-France Six Nations match.
Those affected were urged to make their own way to the game as Northern Ireland Railways said it could not guarantee getting them there by kick-off, despite having called in buses from all over the North to ferry passengers around the 40-mile stretch of closed track between Portadown, County Armagh, and Dundalk.
An RUC spokesman said a number of explosions was heard in the vicinity of the railway line at Newry during the early hours.
Dissident republicans have mounted a renewed campaign against the line recently.
It only fully re-opened a few days ago after a five-day security operation on the line outside Newry found the site of a bomb crater beside the track caused by a device containing an estimated 100lbs of home-made explosives.
On Friday, services were disrupted for a time following a bogus warning of a device further up the track between Moira and Lurgan, County Armagh.
The security operation will continue tomorrow.