Dart services face possible disruption next week because of a dispute between Iarnród Éireann management and unions over whether drivers should be paid more for operating new eight-carriage trains.
Testing is due to begin on Monday on the new longer Darts, which are being introduced following a €250 million investment programme.
However, Siptu and the National Bus and Rail Union, which represent the company's 84 Dart drivers, are seeking talks on compensation for their members for the change.
Siptu branch official Willie Noone said drivers had a "genuine grievance" and wished to have recognition for the extra responsibility and workload from moving from six-carriage to eight-carriage trains.
However, Iarnród Éireann said the Labour Court had already ruled that the drivers were compensated for the move in a major pay agreement five years ago.
The agreement stated that "all opportunities to enhance or to adjust the service in line with customer demand and market opportunity will be taken without resistance or interruption" and that staff would "co-operate with the introduction and upgrading of new technology as required, eg introduction of new or updated traction units ", the company said.
Urging the unions to accept the court ruling, it added that drivers were now earning about €48,000 annually for a five-day 41-hour week.
A spokesman for the company said it had received no formal notice of industrial action but there were indications of a possible boycott of training next week.
"The potential is there that if drivers are refusing to work it may escalate and affect regular Dart services from next week," the spokesman added.
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