The train line between Lurgan and Moira has been closed.
It follows the discovery of a suspicious object near the line in the Cornakinnegar Road area of Lurgan.
An examination of the object is now underway.
Translink says the line will be closed until further notice due to the alert.
A bus substitution will operate between Belfast and Newry in both directions for Enterprise passengers.
“Delays and disruption can be expected,” said a Translink spokesperson.
There are no further details at present.
