SIPTU members in Iarnród Éireann have voted 53% to 47% to reject an amended Labour Court Recommendation on a cost cutting plan for the company.
In early June Iarnród Éireann warned staff that if intensive negotiations failed to secure agreement on cost reductions, it would unilaterally implement the cuts set out in the Labour Court recommendation.
The recommendation was rejected by SIPTU and the National Bus and Rail Union.
It includes temporary pay cuts for 28 months ranging from 1.7% to 6.1%.
Other unions at the company, the TSSA, the TEEU and Unite, accepted the Labour Court recommendation, but the overall ballot result was 51% against, 49% in favour.
The NBRU, which voted no by 75%, has not yet balloted for industrial action.
The amended proposals emerged from discussions between SIPTU and management on an earlier Labour Court recommendation that was previously rejected by the union's members.
SIPTU Organiser Paul Cullen said: "It is clear from this ballot result that our members have categorically rejected the acceptance of any further pay reductions."
He added that the vote means the union does not have a mandate from its members to engage in any further negotiations with Iarnród Éireann.
This latest ballot was conducted from the 19 June to 10 July.