Iarnród Éireann locomotive and DART drivers are to be balloted for all-out strike action over the company's insistence that drivers must make themselves available to mentor new recruits.
Up to now, mentoring has been voluntary, but for almost two years the drivers have refused to carry out this role.
This has resulted in a backlog of trainees unable to complete the final phase of their training.
The company has previously said that this has also affected plans for service improvements, including a proposed ten-minute DART service.
The Labour Court had recommended a pay rise of 1.15% in return for taking on the mentoring duties, but the drivers rejected the recommendation some months ago.
Management subsequently told unions it was proceeding to pay the 1.15% increase, but that from now on the mentoring duties will be deemed compulsory.
The NBRU accuses the company of an unprecedented attack on long-standing industrial relations procedures by forcing changes to terms and conditions regarding training without agreement.
It is understood that the two sides met last Thursday but failed to resolve the row.
NBRU General Secretary Dermot O'Leary queried who was in charge at Iarnród Éireann, given that the current CEO has already announced that he is leaving to take up a post in Australia.
He stressed that the strike would be an all-out indefinite stoppage until the dispute is resolved.
It is also understood the union had been prepared to consider an interim solution - which could encompass the unions' demand for a shorter working week.
However, Mr O'Leary accused the company of ignoring the unions' proposal and indicated that they were proceeding without agreement.
The NBRU has written to the Workplace Relations Commission requesting an immediate intervention.
SIPTU Industrial Organiser Paul Cullen said that once the management had moved to implement unilateral changes in terms and conditions, the unions were left with no option but to ballot for industrial action.
'Intolerable' pressure on driver resources
Iarnród Éireann said service expansion and career progression for more than 30 trainee drivers had been delayed for 21 months due to "trade union non-cooperation", with pressure on driver resources now "intolerable".
In a statement, it confirmed that this had forced it to repeatedly defer the introduction of an increased ten-minute DART frequency, as well as expansion of other commuter services, at a time when passenger numbers reached record levels of 45.5 million in 2017.
The company said it had engaged "repeatedly and extensively" with unions over the last two years at both the Workplace Relations Commission and the Labour Court, but the resulting Labour Court recommendation was rejected.
Last month, Iarnród Éireann told unions all elements of the Labour Court recommendation will now be implemented - including a 1.15% pay rise - on top of the general increase of 7.5% over three years due to the entire workforce.
The allowance for driver mentoring will also rise by one third.
The company has said it has now advertised internally for drivers to express an interest in participating in mentoring to enable training to resume, and urgently needed service expansion to be delivered.
It concluded by warning that industrial action would damage the interests of commuters and drivers alike and it called for urgent engagement with the unions.
Sources at the Workplace Relations Commission said the WRC was evaluating the situation at Iarnród Éireann and was likely to respond to the parties in the coming weeks.