The latest nightmare comes over its newly acquired inter-city trains, which cannot be used after the end of the year unless they have been upgraded to meet disability regulations.

Only three of the 26 “High Speed Trains” (HSTs) have been refurbished and ScotRail chiefs have privately conceded they will not all be ready in time.

Even those first ones arrived late and the entire £54 million overhaul should have been completed by December.

ScotRail MD Alex Hynes is facing a train shortage crisis, as HSTs will not be able to run after December if they don't meet disability regulations. Photograph: Jon Savage

ScotRail is now exploring ways to plug the potential gap, such as trying to find alternative trains.

The move could see a repeat of the operator having to draft in a temporary electric fleet to cover for delays with 70 new trains arriving from manufacturers Hitachi.

However, the headache could be worse this time because there are far fewer diesel trains available.

The 40-year-old HSTs will provide much-needed extra seats to reduce overcrowding between Edinburgh/Glasgow and Aberdeen/Inverness.

ScotRail has been forced to operate ten of them in a “classic” state because of refitters Wabtec struggling with the work, such as replacing old-fashioned manual doors with electrically operated ones.

Robert Samson, of passenger watchdog Transport Focus, said: “It’s imperative that as many of the trains are refurbished as possible. Passengers will want to know what the alternatives will be if they cannot be used after December.”

A Transport Scotland spokesman said: “Ministers and ScotRail have been clear to Wabtec they must improve delivery of the refurbished HSTs so more passengers across inter-city routes can realise the benefits of the huge investment made in the upgraded fleet.

“As with other operators of these trains, ScotRail is also looking at contingency plans.”

A spokeswoman for Angel Trains, which leases the trains to ScotRail, said: “We are undertaking a detailed review of a revised programme from Wabtec. Our aim is to deliver 26 trains as soon as possible. We continue to explore every possible contingency.”

A ScotRail spokesman said: “The refurbished trains are very popular with our customers and we are looking forward to Wabtec delivering all 26 as soon as possible.

“As with the introduction of any trains to our fleet, we are exploring contingency plans to protect the service we provide to customers.”

ScotRail faces another train shortage as upgrades delayed