SIPTU members at Iarnród Éireann have voted to take industrial action if the company moves to unilaterally implement cost reduction measures including pay cuts.
While they backed industrial action by 2 to 1, they rejected the option of full strike action by the same margin.
SIPTU represents around 2,000 of the company's 3,760 staff.
Members had previously balloted to reject a Labour Court recommendation which acknowledged the need for savings including pay cuts ranging from 1.7% to 6.1%.
However, the margin of rejection was very narrow at 51% in favour and 49% against.
75% of National Bus and Rail Union members have also rejected the cost saving proposals but they say they will not be rushed into taking industrial action.
Other unions at the company have accepted the proposals.
A meeting between Iarnród Éireann management and unions to discuss the ballot outcome has ended.
A spokesperson for the company said they noted the outcome of the ballots by unions on the Labour Court recommendation regarding pay and productivity.
The company said the recommendation was in the context of the ongoing financial crisis facing the company and the urgent need to secure payroll savings as part of the overall effort to prevent financial insolvency.
Iarnród Éireann said it will contact the unions in the coming days to advise on its proposed course of action.
Last year Iarnród Éireann had losses of €16.4m.
Its Government subvention has fallen by 38% since 2008, and the Government has warned that no increase in subvention is likely.
Iarnród Éireann has previously warned that if the Labour Court cuts are not implemented, the company could face compulsory redundancies and line closures.