The main union at ScotRail is to press ahead with a threatened ballot to ban overtime that it warned would cause "massive" disruption to passengers.
It followed a meeting with ScotRail on Tuesday that broke up without agreement.
The Rail, Maritime and Transport (RMT) union said ScotRail had been notified and ballot papers would be sent out, with the result expected in about three weeks.
The union then required to give a week's notice of any action.
The dispute centres on train conductors and other staff represented by the union earning less than half as much as drivers for working on days off.
They receive £112-140 compared to drivers' £300 for "rest day working".
The Scotsman revealed the ballot plans last month, but the vote has since been put on hold pending further talks.
RMT regional organiser Scotland Michael Hogg has claimed the impact of banning overtime and rest day working would be massive because he said ScotRail was short of conductors and other staff.
He said: "Nothing was agreed that satisfies the RMT.
"Rest day working payments has failed to be addressed."
ScotRail’s head of customer operations Phil Campbell said on Friday: “We are disappointed by this, but will continue to work with the trade unions and our people to try to reach a solution that benefits everyone.”
Meanwhile, the Unite union said bus engineers at First Glasgow were moving towards industrial action ballot over holiday pay.
It came after talks collapsed at the conciliation service Acas.
Unite said 180 craft and engineering workers were likely to be balloted.
It said Glasgow's main bus operator had been unwilling to retrospectively apply an agreement from 2015.
Unite regional officer Pat McIlvogue said: “The double standards and deceit displayed by First Glasgow is scandalous."
“Unite engineering members are being treated as second class despite a minimum standard framework agreement signed by company directors stating all workers should be treated the same.
"The company is in our opinion operating outside the law on holiday pay and their own agreement.
First Glasgow managing director Andrew Jarvis said: “After a series of talks with our Unite trade union colleagues facilitated by Acas, we regret to advise that despite considerable movement on the established positions of both sides, we have been unable to reach a collective agreement on a revised position on the level of holiday pay paid by the company.
"The company’s final offer was above the level prescribed by recent case law surrounding changes to holiday pay calculations.
"There have been considerable developments in case law in the last three years which were fully satisfied by our final offer to the union."