Dissident republicans are believed to have planted a 50lb bomb that partially exploded on the main Dublin-Belfast railway line, security sources say.
The explosives, contained in a metal case, were discovered on the railway track at Dunmurry, Co Antrim. Damage was slight.
The sources say the alert began on Sunday when a call was received from someone claiming to be from the IRA. The line was reopened on Monday evening.
An RUC spokesman said yesterday: "Following a search and clearance operation police can now confirm that a 50lb bomb contained within a metal case had partially exploded close to the Blacks Road railway bridge."
Mr Jeffrey Donaldson, the Ulster Unionist MP for Lagan Valley, described the incident as "a very worrying development".
"It demonstrates again that republican terrorists have the capacity to deliver their terrorism right to the heart of society. It is clear that they are stepping up their campaign of violence. It is time for the government to get tough with them," he said.
A spokeswoman for Translink, the company that runs Northern Ireland Rail, said the security alert caused traffic chaos in Belfast on Monday.
"We had passengers arriving at Central Station hoping to go on to their place of work, and their plans were thrown into chaos," she said.
Buses were provided to transport passengers, who included schoolchildren and tourists as well as commuters, around the affected area.
In May 1998 a device partially exploded on nearby track at Finaghy. That attack was linked to the Continuity IRA.
