A loaded shotgun was found inside a Luas worker's locker, the Herald can reveal.
The lethal firearm was discovered at the Luas Depot near the Red Cow Roundabout in the west of Dublin.
The Herald understands the shocking discovery of the shotgun was made during a maintenance inspection at the facility where Luas workers are based.
Gardai confirmed that officers in Clondalkin are investigating the discovery at the facility on Saturday, November 21 at 3.30pm.
The locker in which the gun was found was unnamed and had not been used for some time.
"This isn't somebody that hasn't used a gun before," a source told the Herald. "Somebody cut the gun properly.
"The gun had more than 20 rounds of ammunition," the source added.
A spokeswoman for Luas confirmed that a firearm had been found.
"They (maintenance workers) were checking lockers to see which was full and which wasn't to make way for people," a spokeswoman said.
"It was a regular locker check being carried out. The guards were called. This is very much a garda matter.
"There will be no more comment from us," the spokeswoman added.
Luas refused to comment on whether there are any procedures in place regarding the search of staff, lockers or their property.
The shocking find comes as the Government welcomed the order of new trams for the Luas Cross City (LCC) project.
Transport Infrastructure Ireland has revealed that an order for seven Citadis trams has been made.
It is expected the trams - worth more than €36.5m - will be delivered before the Green Line extension comes into operation at the end of 2017.
While the new trams will be the same as those currently being used on the Green Line - they will have two additional carriages per tram - making them a nine-carriage tram.
Each tram can cater for 379 passengers - 60 more passengers than the current vehicles running on the green line.
Transport Minister Pascal Donohoe said the order is an "important step" for the Luas project.
"The new trams will carry more people and allow the Luas Cross City extension to provide a frequent, efficient and integrated transport option for the city centre and all the way through Broadstone and out to Cabra," said Mr Donohoe.
He added that a good plan was put in place between the rail procurement agency and retailers to ensure the construction work causes little disruption coming into the busy Christmas season.
National Transport Authority chief executive Anne Graham said that each passing month is "bringing the project closer to completion".