Thousands of workers at the Network Rail maintenance company are continuing their ballot for strikes against 1,500 job cuts.
The RMT and TSSA transport unions are concerned that safety-critical jobs would be cut.
The ballots end on Friday of next week. A strike could bring the railways to a halt.
The RMT was also to ballot operation staff about them having to take over tasks from maintenance staff.
The ballot is also over the imposition of rosters and the company’s failure to follow the agreed promotion, transfer, redundancy and resettlement arrangements.
Last week a railways inspector served an improvement notice on the company. It highlighted “systemic” failings in track-work safety on lines between Cardiff Central and the Valleys.
But it raises the possibility that “similar failings may be present in other Maintenance Delivery Units across all routes”.
It pinpoints a shortage of lookouts to ensure safe track working. It says these amount to breaches of the Health and Safety at Work Act and railway safety regulations.