The troubled Caledonian Sleeper train service is set to face further disruption, with workers planning to strike on September 29 and 30.
The RMT union called the industrial action due to staff being placed under “intolerable pressure” by train operator Serco.
In a statement, RMT said that “a raft of serious concerns” had “ruined working lives”.
Issues began during the introduction in May of new rolling stock to replace the 40-year-old carriages on the overnight train between London and Scotland.
While advance announcements promised luxurious, hotel-standard cabins, in reality the service has been plagued by difficulties.
This has included understaffing, poor water pressure in the compartments and delayed services.
Problems then escalated to numerous services being cancelled or failing mid-journey, forcing passengers to take the bus in the middle of the night.
Serco has also admitted cleaning failures onboard.
RMT General Secretary Mick Cash said:
“The union is angry and frustrated that promises made to us to address the serious concerns of the Caledonian Sleeper staff at the end of July have not been honoured and that leaves us with no choice but to activate our industrial action mandate.
“RMT members have been put under intolerable personal stress as a result of the company’s mismanagement of the sleeper service. They know it, they have promised to do something about it but they have failed to deliver.
“The move to strike action is entirely SERCO’s fault and the public will understand that. Everyone knows that when it comes to failure they are global specialists.
“The union remains available for genuine and serious talks.”
Caledonian Sleeper managing director Ryan Flaherty told the BBC the company was “disappointed and frustrated” by the threat of industrial action.
“We have met the RMT and company council on many occasions and the changes they have requested have either been completed or are under way,” he said.
These include the recruitment of more staff at Edinburgh, Glasgow and London for a six-month period whilst the new carriages “settle in”.
Flaherty added: “This action is completely unnecessary and we urge the RMT to come back and tell us what they want. We are ready to go to mediation and hold further discussions.”