Police are carrying out searches at the railway line in Lurgan this morning (Wednesday).
The operation is taking place at Lake Street and Kilwilkie Road.
It follows a report of suspicious activity in the area on Sunday.
A spokesman for the PSNI said: “Police are currently carrying out a search at the Lake Street and Kilwilkie Road, Lurgan area this morning. The search is in relation to a report of suspicious activity in the Kilwilkie area on Sunday 27 April. There are no further details at this stage.”
Translink said all lines between Portadown and Lisburn have been closed until further notice while the PSNI investigation is carried out.
A spokesperson explained: “Train services through these stations may be subject to disruption on all routes at short notice. An estimate for the resumption of normal services will be provided as soon as the problem has been fully assessed. Bus substitution is in place between Lisburn and Portadown.”