The National Bus and Rail Union has denied allegations from Irish Rail that Dart drivers are involved in unofficial industrial action.
The company is threatening legal action against the NBRU and SIPTU, after Dart drivers refused to train in new staff.
The NBRU said drivers had the right to refuse to train new recruits, and that the participation of trained drivers in the scheme as mentors is entirely voluntary.
The company claims drivers are in breach of contract, and said they are involved in "unlawful activity".
Irish Rail and the drivers' unions are currently in negotiations over pay.
However, Dermot O'Leary of the NBRU said the union was not involved any coordination of the workers.
"(The company) have no basis whatsoever for attacking this trade union, and no basis for initiating legal proceedings," he said.
"We have not been coordinating any action. Any action we would undertake would be at the end of a ballot or a mandate from our members and that hasn’t taken place and is not likely to take place in the near future."
In an additional statement, Mr O'Leary said: "Irish Rail has reached a new low by threatening staff and trade unions with High Court action over what is clearly a matter of individual choice for drivers.
"This aggressive Thatcherite type reaction does nothing but show how devoid of ideas those who now run this company actually are…It is nothing short of appalling that a state-owned organisation would resort to such skulduggery."
Barry Kenny from Irish Rail said: "Quite clearly, there is an orchestrated action from the trade unions...This is unofficial action. It needs to stop (and) the trade union needs to intervene to ensure that it ends."