Irish Rail is being warned workers may go on all out strike at the company.
The NBRU claims bosses are refusing to return to talks in their long running pay dispute.
The company is attempting to increase Dart frequency and had to postpone the plans in recent weeks.
A ballot on industrial action is underway, with a result expected next week.
The NBRU General Secretary Dermot O'Leary has refuted claims that drivers are blocking a new ten minute service.
“I don’t know what Irish Rail actually want here,” said O'Leary.
“On one hand they’re telling the public at large that we’re blocking a 10-minute (Dart) service, we volunteered and said we’d include a 10-minute service in any discussions.
“All we wanted to do is have a discussion and the company are refusing to go to the WRC.”