Passenger complaints about the Caledonian Sleeper rocketed by 67 per cent after the introduction of its fault-plagued £150 million new fleet, official figures showed today.
The overnight services between Scotland and London also attracted the highest complaint rate of any train operator, the Office of Rail and Road (ORR) regulator reported.
There were 206 complaints per 100,000 journeys between April and June, up from 123 in the same period last year.
The Serco-run trains were introduced between Edinburgh, Glasgow and London in April but have suffered a significant number of technical faults, such as water problems with the new ensuite toilets and showers.
Passengers have also been left without food because of staff shortages, and services have been cancelled by industrial action.
Caledonian Sleeper had the third poorest response rate among operators for complaints, with 96.3 per cent answered within 20 working days - four weeks - although this was 4.3 points better than a year ago.
The company was also the latest in providing delay compensation - only 87.4 per cent of claims were responded to within 20 days, 13 points worse than a year ago.
ScotRail complaints also increased by 35 per cent, to 31 per 100,000 journeys, but it was placed in the middle of the league table of operators.
The two main cross-Border operators were second and third worst in the table - Virgin Trains West Coast and UK Government-run LNER on the east coast.
Virgin notched up 143 complaints per 100,000 journeys, although this was down by 25 per cent on last year.
LNER were up 23 per cent to 127.
The ORR said those companies holding the top three spots "is consistent with previous data and representative of he fact that long-distance operators receive more correspondence about their services".
LNER also had the second worst complaints response rate - after Essex operator c2c - managing to reply to 94.2 per cent within 20 days, which was 1.6 points worse than a year ago.
By contrast, six firms, including cross-Border operators CrossCountry and TransPennine Express answered all complaints with in that period.
ScotRail's score was 99.9 per cent, unchanged on last year
It also responded to all delay compensation claims within the period.
ScotRail said it had been a "challenging period", shortly after the publication of a remedial plan because of poor performance.
It said its service had "greatly enhanced" since then, with improved punctuality and a fall in fines for below-par service quality.
A spokesperson said: “Everyone at ScotRail is working flat out to deliver the service our customers expect and deserve, and it is encouraging to see recent improvements both in performance, and in facilities at our stations and on trains.
“We understand and share the frustration of our customers when things do not go to plan and we welcome the engagement we have with them to help improve their experience.”
Scottish Labour transport spokesman Colin Smyth said: “More and more passengers on the Caledonian Sleeper are waking up to the fact our privatised railways are not being run in their interests.
“Complaints are rising amid a spate of cancellations and delays on the service.
“It is vital for Scotland’s economy to have a reliable and efficient overnight rail link with businesses in London.
“Labour will bring our railways into public ownership so our transport system delivers for passengers, not profits.”
Ryan Flaherty, Serco’s managing director for Caledonian Sleeper, said: “We take all complaints seriously.
"And while complaints at this level are unusual for us, during the launch period of our new trains there were some nights when full trains experienced disruption, which would drive such an increase.
“Our entire team has a relentless focus on improving guest experience - whether in terms of better on-board facilities or reducing delays.
"Good progress is being made and we expect that trend to continue over the coming weeks and months as we strive to deliver the best possible experience for all guests.”