Irish Rail has threatened the NBRU and SIPTU with legal action, after DART drivers with the unions refused to train in new staff.
The company has written to the unions accusing the workers engaging in co-ordinated unofficial industrial action.
The letter claims the drivers are also in breach of their contract of employment.
The NBRU described the company's approach as an "aggressive Thatcherite type reaction".
It claims any decision not to allow new recruits on board was made on an individual basis by the drivers.
It has accused the company of "skulduggery" and says the union only takes industrial action when it gets a mandate from its members.