The railway line between the two towns was closed to commuters due to the operation.
A PSNI spokesperson said: “Detectives from the Terrorism Investigation Unit carried out a search in the Antrim Road area of Lurgan this morning, May 23, as part of an ongoing investigation into violent dissident republican activity.
“The planned search was conducted with the support of Tactical Support Group colleagues.”
Translink put on a bus replacement service for those affected.
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.”