Caledonian Sleeper workers are to vote on whether they should go on strike amid claims staff are suffering "physical breakdowns" due to stress.
RMT is to ballot its members about possible industrial action after the "chaotic" introduction of new rolling stock on lowlander services.
The union blames poor staffing levels for mental health issues being "rife" among the Serco workforce, which has seen employees "left in tears" at the end of shifts, and as revealed by the Scotsman.
General secretary Mick Cash said: "The introduction of these new trains should be a positive move for both our members and the railway industry.
"However, this could not be further from the truth, with the situation now reaching breaking point.
"The union has informed the company that the morale of their employees has collapsed - with some unable to see how they are ever going to have anything like a good work-life balance again.
"This situation is deplorable and the response from Serco has been inadequate, dismissive and has totally broken industrial relations between our two organisations."
RMT is now preparing to ballot Serco Caledonian Sleeper members for strike action and action short of strike.
Ryan Flaherty, Serco Managing Director of the Caledonian Sleeper, said: “We are deeply disappointed and surprised at this move by the RMT.
"We stand ready to discuss the points they have raised and their decision to proceed to a ballot is premature, not in the best interests of our employees and contrary to accepted practice. We would urge the RMT not to disrupt the service for our guests, for some of whom this is a lifeline service.”