Drivers at Irish Rail are to vote on possible strike action.
Unions are accusing the company of forcing changes to training - which is currently done voluntarily.
The National Bus and Rail Union will ballot all train and DART drivers for industrial action as it says that Irish Rail is forcing changes to the training regime without agreement.
Drivers currently train new recruits on a voluntary basis but the union says the company's decision to make changes is an "unprecedented attack on industrial relations".
The Unions have written to the Workplace Relations Commission to request an immediate intervention.