The admission came as opposition members questioned the cabinet minister about passenger complaints with the service soaring by 221 per cent last summer.

Mr Matheson said train manufacturer Caf was working through rectifying a range of problems.

They include a circuit fault with emergency exit windows, which have had to be locked shut, as Scotland on Sunday revealed last weekend.

Passenger complaints about Caledonian Sleeper more than trebled last summer

Other problems have included chemicals damaging pipework, affecting toilets and showers, and software locking out rooms and toilets.

BACKGROUND: Caledonian Sleeper passenger complaints soar by 221%The £150 million fleet operated by Serco was introduced a year late on the "Lowlander" routes between Edinburgh, Glasgow and London.

The faults led to its launch on the "Highlander" routes between Aberdeen,Fort William, Inverness and London being postponed from June to October.

Mr Matheson said: "Not only have I engaged with Serco on this matter, I have also engaged directly with senior executives within Caf around the specific problems which have been experienced with the introduction of the new rolling stock.

"There are a range of measures which have been taken as a result of that.

"There has been a rolling programme of retrospective technical work that has been undertaken on the new rolling stock by Caf in order to close out what have been ongoing issues that have had an impact on the passenger experience.

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"Good progress has been made on that matter, according to Caledonian Sleeper, and they expect that work to be completed in the next couple of months, which will ensure all the technical issues which have had an impact on the passenger experience have been closed out.

"The picture is one of improvement."

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He said passenger journeys had increased by 20.8 per cent since last October, when the new trains started operating all routes.

Scottish Labour transport spokesman Colin Smyth, who launched the questioning at Holyrood, said: “This is another blow for the Caledonian Sleeper franchise which has been plagued by problems from the start.

"We’ve had trains overshooting the platform, services introduced in the north a year late, staff walking out due to stress levels and now emergency exit windows that won’t open.

"It is little wonder the latest complaints figures are up 221 per cent and performance levels have fallen."

Magnus Conn, Serco’s operations director for Caledonian Sleeper, said: “We continue to work closely with Caf on outstanding issues with our new fleet of trains.

"Since their introduction last year, we have seen significant improvements and fully expect that to continue in the weeks and months ahead.

"Indeed, we have already seen an increase in our right-time arrival performance [within one minute of schedule], with the last full period showing a figure of 82.9 per cent against a target of 80 per cent, and the current period sitting at 85.7 per cent.”

Caledonian Sleeper faults will take another two months to fix