Police have been attacked with petrol bombs as they tried to remove barricades from the railway line in Lurgan, County Armagh.
The material was found during police searches
A number of people gathered in the Bells Row area after a hijacked van was driven onto the line and set on fire earlier on Thursday.
Police left the area and said they were talking to community representatives.
The town's Antrim Road and the railway line were closed for a time, but have now re-opened.
Earlier on Thursday, more searches were carried out in the town following the discovery of a bomb police have linked to dissident republicans. There were no arrests.
Four people are still being questioned about the discovery of components for the 250lb car bomb - found in the Antrim Road area - on Wednesday.
The men, aged 22, 26, 36, and 46, are being held under the Terrorism Act.
Police officers also came under attack from youths during Wednesday's security operation.
Dissident republicans opposed to the Northern Ireland peace process
have been blamed for a number of attacks and attempted attacks on the security forces over recent years.