They would have to be pressed into service in December as part of the operator’s biggest increase in services for 25 years, The Scotsman has learned.
The 40-year-old trains should have started on routes such as Aberdeen-Edinburgh in May.
However, the first of the 26 trains is not expected to carry passengers until next month.
ScotRail needs ten for the extra services and expects it will have to use up to nine unrefurbished trains being used to train drivers.
Managing director Alex Hynes said: “We will struggle to have the trains by December. [Refurbishment firm] Wabtec’s delivery has been dreadful.
“The amount of work needed has been more than they budgeted for.
“We may have to operate them in ‘classic mode’ - in an unrefurbished state.”
However, he said passengers would prefer that to keeping the current trains because they were more comfortable, smoother and quieter.
Scottish Liberal Democrat transport spokesman Mike Rumbles said: “The public ought to be able to expect better than temporary stop-gap solutions.”