Iarnród Éireann management and unions have begun a week of intensive negotiations, in a bid to reach agreement on cost reduction measures including pay cuts.
However, the company warned employees that if agreement is not reached, it will implement the measures on a unilateral basis.
SIPTU has already voted for industrial action if that happens, but members stopped short of backing full strike action.
SIPTU and the National Bus and Rail Union have rejected a Labour Court recommendation, which found that the measures were essential to protect the viability of the company.
However, the recommendation had shortened the duration of pay cuts from three years to 28 months.
While SIPTU voted against the proposals by a narrow margin of 51% to 49%, NBRU members rejected them emphatically by three to one.
Other unions at the company have accepted the Labour Court recommendation.