The General Workers' Union will be giving Arriva a 48-hour ultimatum to return to the negotiating table to discuss an improved financial package for workers.
It said the decision was taken following two meetings with employees. Meetings with other employees are to be held shortly.
The union said in a statement that unless Arriva returned to negotiations, it would be free to register an industrial dispute and take the action it deemed suitable to defend Arriva employees.
The GWU said it will be informing Arriva of its position once all scheduled meetings with workers are concluded.