The four unions said they were “furious” with Scottish Rail Holdings (SRH) chief executive Chris Gibb at being told staff conditions would be downgraded. as part of the switch.
One official told The Scotsman they expected it to lead to an industrial action ballot.
The outburst bodes ill for the transfer of ScotRail after being in private hands for 25 years, although the unions will have sought to impress on the new regime their importance and vowed to fight any perceived reduction of their members’ conditions.
The unions were told that ScotRail staff would be subject to Scottish public sector pay and conditions policies.
They claimed there would also be an end to the no compulsory redundancies policy, which was part of the franchise agreement with current operator Abellio, but the Scottish Government’s Transport Scotland agency disputed that.
The unions said they were also informed of “further plans to cut their right to negotiate over pay and conditions”, including limits to pay increases.
But one observer said the unions appeared to want ScotRail to be part of the public sector without having to abide by public sector policies.
The bad blood follows a long-running dispute last year over pay that halted many Sunday trains for months and threatened to cause major disruption during the COP26 climate change summit in November.
In a joint statement, the Rail, Maritime and Transport union, drivers’ union Aslef, the Transport Salaried Staffs Association, which represents office workers, and engineers’ union Unite, accused Mr Gibb of a “confrontational” approach.
They pointed to plans to close or reduce the opening hours of 123 station ticket offices, although ScotRail has said staff would be redeployed with no redundancies.
The unions said: “Scotland’s new rail operator has started their relationship with workers in a more confrontational and provocative way than any of their failed private operator predecessors.
“To be threatened with compulsory redundancies and cuts to pay and conditions is an attack on key workers who have kept Scotland moving during the pandemic.
"We are bitterly disappointed that rather than meet in good faith and seek to rebuild industrial relations, Mr Gibb seems intent on repeating the mistakes of the past."
Transport Scotland said there were no current plans for compulsory redundancies and there was scope for additional pay increases linked to productivity improvements.
Its spokesperson said: “We do not recognise this interpretation of the meeting and it is disappointing the union leaders have taken this approach, which will cause misunderstanding and distress for their members.
“Conversely, we welcome the recent engagement with the trade unions regarding the formal transfer to ScotRail Trains on 1 April.
"We are keen to ensure every current member of ScotRail’s staff wishing to transfer to the new publicly-controlled company with their current terms and conditions does so.
“Ongoing Transfer of Undertakings (Protection of Employment) discussions will keep staff informed of all matters related to the transfer, including the ways public sector pay policy could be applied.
"This is a pay policy which continues to focus on sustainability, reducing inequalities and promoting well being in an extremely challenging fiscal context.”