The PSNI says the train line between Lisburn and Portadown has been closed due to a "continuing security operation" in Lurgan.
Irish Rail says Enterprise services are operating a bus transfer between Newry and Belfast.
Delays of up to 70 minutes are expected on the Dublin Connolly to Belfast service.
Dublin Connolly - Belfast, significant delay of approx 70mins. to 16.50hrs. service due to security alert.
— Iarnród Éireann (@IrishRail) August 5, 2016
While Northern Ireland Railways says passengers can expect disruption until further notice.
Buses have been requested to provide transport for cross-border services.
NI Railway tickets will be accepted on Ulsterbus services operating to/from Lisburn and Portadown during the disruption.