Union bosses have criticised plans recruit long-term convicts in voluntary roles instead of hiring full-time staff.
The train provider are in discussions with the Scottish Prison Service over day-release schemes for long-term convicts.
Voluntary roles – including cleaning duties – would be built into the inmates’ training for freedom programme.
Offenders recently released from custody would also be drafted in to the positions.
But the Rail Maritime Transport (RMT) union say the under-fire rail operators should be hiring full-time staff for the jobs.
They say there are up to 90 vacancies at ScotRail across areas such as on-train hospitality, sanitary and gate workers.
The union also voiced anger at the number of agency staff being used as short-term cover for vacant positions.
Inmates are due to take up posts at major stations including Glasgow Central and Edinburgh Waverley.