Armed gardai have arrested a man and seized a firearm on the Dublin to Belfast train as it prepared to leave Connolly Station.
The incident took place around 3.20pm on Wednesday and the suspect being questioned at Store Street garda station.
The 20-year-old has been detained under Section 30 of the Offences Against the State Act.
It is not yet known if the incident is terrorist related.
The train was delayed for 15 minutes while police carried out the operation.
A Translink spokeswoman said: “A person on the 1520 Enterprise train was arrested by gardai before the train departed Connolly Station in Dublin.”
One eyewitness, who was travelling to Dundalk, told dublinlive.ie: “It was unreal. We thought what was going on was a joke.
“About 20 undercover guards and Armed Response Unit guards all came onto the train.
“I’m still a bit shocked at the moment.”