ScotRail has insisted industrial action by some of its driver managers from tomorrow will have "absolutely no impact on services".
The Transport Salaried Staffs' Association (TSSA) said the eight staff it represents among some 50 ScotRail driver team managers will begin indefinite action.
The union is in dispute with ScotRail over the managers having to step in to drive trains when there is a shortage of drivers to prevent services being cancelled.
It said services around Edinburgh were likely to be affected because all the driver team managers based in the capital were TSSA members.
The union said switching managers to driving trains would affect ScotRail's driver training programme "and cause more problems for ScotRail and their passengers down the line"
TSSA leader Manuel Cortes said: “ScotRail know that from Thursday, trains will be cancelled as our members exercise their lawful right to take part in industrial action, but rather than engage in meaningful negotiations, they chose to issue threats to our members.
“This is entirely a dispute of their making.
"Firstly, by failing to recruit enough drivers, in an effort to make a quick profit.
"When ScotRail realised they’d need our members to make up for their shortfall, they chose to impose changes on our members rather than negotiate with them.
But a ScotRail spokesperson said: “We have had numerous meetings with TSSA representatives and we are absolutely committed to engaging with them to reach a resolution.
“ScotRail values each and every member of staff and their hardwork to deliver a high standard of service for our customers.
"Any suggestion of bullying is completely untrue and it is very disappointing that the TSSA would wilfully misrepresent our efforts and mislead their members in this way.
“The dispute itself affects only seven people in a company with over 5,000 employees, so any industrial action will have absolutely no impact on services.”
ScotRail said driver team managers were already required to drive trains regularly to help maintaining their skills.
It said it was hiring 55 more drivers this year.