The Belfast-Dublin railway line was blocked again yesterday by disturbances following the discovery on Wednesday of a suspected bomb factory in Lurgan, Co Armagh.
Masked men drove a van on to the line before setting it alight. The disturbances followed further searches carried out by the PSNI. There were no additional arrests and the police have said they have several items for examination, including a car and a van.
The four men arrested on the morning of the discovery of bomb components and more than 100kg of fertiliser-based home-made explosives, are still in police custody.
They include a well-known businessman who is well-respected and not known for any links to any paramilitary group.
Three of those arrested were detained at the scene in the early hours of Wednesday.
The fourth suspect was taken into custody at another location hours later.
All four are being held under the Terrorism Act and are being questioned at the serious crimes suite at Antrim PSNI station.
The PSNI said the bomb was at an advanced state of construction and would have been ready for imminent placing at an unknown target in the area. The device was unstable, they added.
They also believe the Continuity IRA was behind the incident.
Minister for Justice Michael McDowell yesterday branded whichever organisation was responsible as "fanatics" who were determined to thwart any political progress.
Speaking at the Garda college in Templemore, Co Tipperary, he said: "There are two main dissident republican groups, and both of them are attempting to disrupt the peace process and to somehow blow it off course.
"They are also intent on creating death and destruction in Northern Ireland.
"They believe in some mad way that this will advance the cause of a united Ireland," Mr McDowell said.
Commending both An Garda Síochána and the PSNI, Mr McDowell said: "The gardaí and the PSNI on a number of occasions have foiled very dangerous massive bombs which have been constructed by both of these groups.
"All I can say is that I want to congratulate the PSNI for intervening on this occasion and congratulate both police forces of this island for their strong support for the rule of law and for what the Irish people voted for, which is peace," Mr McDowell added.