A union fears it will mean passengers on overcrowded inter-city ScotRail routes having to wait until the end of the year for trains which were originally due to have been introduced in March.
Problems are believed to include the fitting of the wrong carpets, which have had to be ripped out.
News of the further delay to the former InterCity High Speed Trains (HSTs) comes days before ScotRail plans to start running the first of its brand new electric trains – ten months late. The veteran expresses, designed by Sir Kenneth Grange, hold the diesel speed record of 148mph set in 1987. They are being phased out from the Great Western and East Coast main lines where they started operating in 1976.
However, 26 are being overhauled at a cost of £54 million to provide more space and comfort – including hot meals – on routes between Edinburgh, Glasgow, Aberdeen and Inverness.
They are expected to run until 2030.
ScotRail had planned to launch them this month, but it is understood the first will not now be ready until at least the end of the summer because of delays at the Wabtec works in Doncaster.
Kevin Lindsay, Scottish secretary of train drivers’ union Aslef, said it could be as late as December.
ScotRail chief operating officer Angus Thom said: “We had hoped our suppliers, [trains owner] Angel Trains and Wabtec, could have delivered the first InterCity train to our deadline.
“Their delays prevent us from beginning to transform how we connect customers across Scotland’s seven cities.
“Our suppliers have faced challenges with their delivery schedule, and we are working closely with them to get these iconic trains running as soon as we can.”
An Angel Trains spokeswoman said: “Due to unforeseen refurbishment issues in the supply chain, the delivery of the HSTs to ScotRail has been delayed. The introduction of the first HST on the Aberdeen-Edinburgh route is forecast for later this summer.”
A Transport Scotland spokeswoman said: “While any delay in the programme is frustrating, we are sure passengers will welcome the service improvements they bring when they undergo a phased introduction in the coming months.”
No Wabtec response was provided.