Police have found firearms and munitions, including a mortar explosive, during an ongoing two-day search operation.
The arms were found in a wooded area near a derelict house on the outskirts of Lurgan, Co Armagh.
Police believe one of items was a "fully constructed explosive device".
Detective Superintendent Karen Baxter said: "These items were in the advanced stages of readiness and some were ready to be used."
She said it was a "significant and key" find that had saved lives.
Other items included pipe bombs, homemade explosives and three firearms.
The train line, closed between Lisburn and Portadown, due to security operation has now now re-opened.
On Wednesday the railway line was closed for up to six hours while a major police search operation took place in Co Armagh.
The line was closed from mid-afternoon while the PSNI carried out searches in the Cornakinnegar Road area of Lurgan.
Doug Beattie MC, Ulster Unionist MLA for Upper Bann and Ross Hussey MLA, Ulster Unionist Policing spokesperson, praised the PSNI for their work.
Doug Beattie said: "This is fantastic work by the PSNI in the Lurgan area. The find of a substantial amount of arms & munitions is a major success and a significant blow against republican terrorism.
"Some of these devices were in the advanced stages of construction. Without a shadow of a doubt, their removal has saved individuals from death or serious injury."
Ross Hussey said: "This is great police work and good news for the people of Northern Ireland. Another cache of weapons that could have been used to bring death and destruction has been taken out of circulation by the police.
"As the police operations continue, I would encourage anyone with information to pass it to the PSNI."
Upper Bann DUP MLA Carla Lockhart said: "Lives have been saved thanks to the actions of the PSNI, who have been carrying out this two day search operation on the outskirts of Lurgan.
"Dissident activity has been thwarted. It once again should be a wake-up call to those in our community who want to return to the past, that the PSNI will continue to work to defeat the threat of dissident activity. Their actions have no place in our community.
"I understand that major firearms and ammunitions have been found and there is no doubt that the intention for these weapons was to kill, mane and disrupt everyone’s way of life and that is extremely worrying to those living in the area. I would urge anyone who may have information to get in touch with Police to help with their on-going investigations. I would also ask the public to remain vigilant and to report any suspicious activity.”
Upper Bann DUP MP David Simpson added: "This a huge and significant blow against dissident Republicans who want to draw Northern Ireland back into the dark days of terrorism. The security forces are to be congratulated in their work for uncovering this and I hope they will continue to make significant strides against terrorism here."