Caledonian Sleeper staff are to be balloted for industrial action over “potentially dangerous” defects with the ageing trains.
The move by the Rail Maritime and Transport union (RMT) follows claims of faulty smoke alarms, lighting and toilets.
The union said there was growing discontent among the 100-strong workforce over the problems, which have also left passengers without hot water.
RMT Scotland regional organiser Gordon Martin said there were currently more than 200 defects with the trains, which had led to complete carriages being closed on some services.
Some coaches in the fleet, which is to be replaced in 2018, date back more than 40 years.
The trains run six nights a week between Edinburgh, Glasgow, Aberdeen, Fort William, Inverness and London.
The RMT, which represents nine in ten Sleeper staff, said the number of faults had “gone through the roof” since Serco took over the franchise from FirstGroup in March.
It blamed that on Serco switching maintenance from in-house to another firm, Alstom.
An RMT spokesman said: “Some of the defects are potentially dangerous to the staff on board and the travelling public, who are paying top dollar prices for a service which is unreliable, increasingly uncomfortable and riddled with health, safety and welfare issues.”
General secretary Mick Cash said: “The poor service to passengers, in conjunction with the company’s failure to address the issues raised by the RMT over several months, has now reached the point where the union has decided enough is enough and we will proceed to ballot for industrial action before someone is injured.”
The union is also campaigning against Serco’s bid to take over the public-sector held CalMac ferry contract.
Caledonian Sleeper managing director Peter Strachan said: “We are disappointed the RMT is considering industrial action and are absolutely committed to continued dialogue with our employees and their representatives.
“Since taking over running of the Caledonian Sleeper eight months ago, Serco has worked hard to improve the service for both our customers and staff.”