The National Bus and Rail Union has warned Irish Rail that it should be careful about taking legal action against its members.
Iarnród Éireann indicated on Monday it will take High Court action against the NBRU and Siptu over what it alleged was unofficial industrial action by Dart drivers.
It maintained that Dart drivers had engaged in co-ordinated action in refusing to train new recruits.
Dermot O’Leary, spokesperson for the trade union, told Newstalk Breakfast that it was disgraceful that a state funded company was legally threatening workers representatives.
“Industrial relations should never play out in the legal arena. There are plenty of other forums for that. There seems to be a reluctance at higher levels in Irish Rail to engage in these forums.
"Our 3,500 members work across Dublin Bus, Bus Éireann and Irish Rail. They should be careful before they start issuing threats."
Barry Kenny of Irish Rail said that the company had written to the union calling for unofficial action on trainee drivers to stop.
“We are training a new group of drivers in cab familiarisation, they are just there to observe the cab environment.
“We attempted to get this phase under way, but there is an orchestrated action by the Trade Union, an unofficial action to prevent this happening.
“The company has written to the unions saying this is unofficial action and has to stop.
“The legal option is there for us to take an injunction. The letter was to the Trade Unions not to employees.
“Clearly when you have 23 drivers declining to take trainees onboard, this is something orchestrated by the union,” said Mr Kenny.