SIPTU has joined the National Bus and Rail Union in threatening industrial action at Irish Rail.
Both unions say there is growing frustration among train drivers over carrying out additional duties, without extra pay.
SIPTU is accusing the company of stalling discussions at the Labour Relations Commission regarding compensation for extra duties.
"The point of where we're coming to now...really shows how industrial relations is being dealt with in Irish Rail," said SIPTU's Paul Cullen.
"There seems to be a long, protracted, drawn-out discussion in relation to all of the issues and fudging of the issues."
However, Barry Kenny of Irish Rail insists that the ball is in the unions' court.
"We wrote to the trade unions last year looking for proposals in that regard," he said.
"We have since put forward our own proposals and received responses on that, and we're in a process with the LRC, which should continue, rather than have industrial action threatened to our customers."