Iarnród Éireann has threatened to take legal action against SIPTU and the National Bus and Rail Union alleging that drivers have breached their contracts of employment by refusing to cooperate with training of new drivers.
The move comes as unions and the company are locked in a dispute over the introduction of a ten minute DART timetable which has been shelved due to the failure to reach agreement.
The warning comes in a letter from Iarnród Éireann Director of Human Resources Ciaran Masterson to SIPTU and the NBRU.
He claims that over the past two days, drivers refused "en bloc" to allow newly recruited drivers to travel with them in their cabs as part of their training.
He described this as a clear breach of the contract of employment, adding that "such action has obviously been coordinated and is a clear example of unofficial industrial action in breach of the Industrial Relations Act 1990 (as amended)."
He called on the union officials to immediately inform their members to cease what he called "this unlawful activity", saying failure to do so will result in action, including legal proceedings, will be taken against SIPTU and the NBRU.
In a replying letter, the NBRU's solicitors denied the company's claims that the union had coordinated any unofficial industrial action.
It said that under the contracts of employment and previous correspondence from management, cooperation with in cab familiarisation is voluntary.
However, they reiterate that the union is prepared to return to the WRC for talks.
In a subsequent statement, NBRU General Secretary Dermot O'Leary accused Irish Rail of reaching a new low, saying it was nothing short of appalling that a State-owned organisation would resort to such skulduggery.
He said: "The NBRU will vigorously defend our integrity as a trade union, any industrial action we ever undertake, commences only after we receive a mandate from our members.
The union has also written to all TDs outlining their view of the dispute, and urging them to commit to workers' rights.