Train passengers in Dumfries and Galloway could be hit with a double whammy of strike action in September.
Members of the National Union of Rail, Maritime and Transport Workers (RMT) and Unite are in separate disputes with operators.
Talks between ScotRail and Unite aimed at resolving the row collapsed and relations have broken down.
The union subsequently launched a ballot for industrial action by its engineering members claiming management had failed to provide a “meaningful” pay offer.
Pat McIlvogue, Unite’s industrial officer, said: “The working relationship between the trade unions and Abellio ScotRail is non-existent.
“It has plummeted to the lowest level in living memory which comes at a time when Abellio’s management of the rail franchise approaches an ignominious end.
“We’re confident our members will vote in favour of taking action.”
ScotRail runs services from Glasgow to Dumfries via Sanquhar and to Stranraer.
If members vote for industrial action – the ballot closes on September 1 – action will take place in “early September”.
That would run simultaneously with RMT’s ongoing six-month dispute with ScotRail over ticket examiners and conductors being paid a lower wage than drivers for working days off.
No Sunday services run in the region as part of strike action which started in March and will continue into next month.
RMT general secretary Mick Lynch said: “For six months now our members have stood strong on ScotRail in this fight for pay justice and equality and they are as united now as they were right at the start of what we believe is now Britain’s longest-running industrial dispute.
“We continue to press Transport Scotland, the government and Abellio to break their silence, get out of the bunker and sit down with us to negotiate a fair and just settlement.”
ScotRail will come back into public ownership when the franchise agreement with Dutch firm Abellio runs out in March.
A company spokesman said: “It’s disappointing that Unite is balloting members on industrial action just as we are starting to see customer return to Scotland’s Railway.
“Strike action is wrong at a time when we need to deliver a safe and reliable service and provide stability for our staff and customers alike. It is frustrating Unite have tried to disrupt our services instead of working together with ScotRail to address the longer-term challenges that face the railway.
“We are seeing a gradual increase in the number of customers returning to the railway but, with passenger numbers at only 50 per cent of the pre-Covid level, this is not the time to put that recovery in jeopardy.
“Railway jobs are being put at risk by the reckless actions of the RMT.”