The train lines between the two towns, also affecting Moira and Lurgan, were closed for some time this morning’.
A PSNI spokesperson said: “Detectives from the Terrorism Investigation Unit carried out a search in the Antrim Road area of Lurgan this morning, 23 May, as part of an ongoing investigation into violent dissident republican activity.
"The planned search was conducted with the support of Tactical Support Group colleagues,” said the PSNI spokesperson.
One commuter said: "I was on the train from Belfast when we were told we wouldn’t be going any further than Lisburn,” said one commuter.
The commuter, who had been attending an appointment, said Translink were putting on a bus service to bring them to their destination.
He said staff on the train told passengers that the PSNI was dealing with the incident but had not given them any details of what the incident was about.
A Translink spokesperson said: “Following an earlier closure the line between Lisburn and Portadown has re-opened. Please allow for delays/disruption while services get back to normal.”
Previously the spokesperson said: “The line between Lisburn and Portadown is closed due to an incident.
"Please allow for disruption/delays to services until further notice. We will keep you updated from here,” she said.
A translink spokesperson said bus substitution service will operate between Lisburn and Portadown due to the ongoing incident.