Irish Rail has said that the blockage to driver training must be addressed by trade unions.
The two sides in the dispute entered talks at the Workplace Relations Commission today.
Drivers are looking for a reduced working week.
They have threatened to go on strike unless their hours are brought in line with train drivers in the UK and Northern Ireland.
This would mean cutting their working week from 43 to 35 hours.
“There is a continuing organised withdrawal of cooperation by drivers with the training of new drivers, which must be addressed by trade unions,” Iarnród Éireann stated.
“Nine trainees in DART are being prevented from completing training, affecting the company’s ability to respond to the needs of customers in the future.
“Already trade unions have withdrawn cooperation with 10-minute DART frequency, and non-cooperation with training has emerged on multiple occasions as we seek to increase the number of drivers to meet future service requirements.”
It added: “These are crucial areas on which we wish to engage, but continuing frustration of training is unsustainable if we are to progress issues of concern to our drivers.”
A joint statement from the National Bus & Rail Union and Siptu read: "Both unions today agreed at the WRC to also discuss the issue of driver training as part of the agenda set by the Labour Court Commissioned Report with the understanding, similar and consistent with industrial relations norms, that nothing is agreed until everything is agreed, we will as a matter of course return to the WRC tomorrow morning in the hope that the company will engage on the comprehensive agenda set out in the report, failure to so do will result in an immediate ballot of our members for industrial action."
Talks will continue in the morning.