A SPATE of breakdowns on Scotland's new flagship railway has been blamed on a fleet-wide fault affecting the route's newly-refurbished Class 158 trains which causes them to overheat in hot weather.

ScotRail is seeking replacement radiators for the diesel trains, which are used on several lines including the Borders Railway, in a bid to iron out the flaw which is believed to have been a factor in at least 18 service cancellations on the new Edinburgh-Tweedbank route over the summer.

The two-car units are notoriously the least reliable in the ScotRail fleet, but industry magazine, 'Rail', reports that the company has linked the latest problem with the Class 158s to hot weather.

Thermal imaging investigations are understood to have revealed that some of the trains' coolant systems become more stressed on hot days, especially when the train is accelerating over a steep gradient, causing the engines to over heat and shut down. Radiators have been blamed for the fault.

Drivers have been issued with new operating instructions in an attempt to mitigate the situation in the short-term, while ScotRail procures new radiators to avoid a repeat of the disruption next summer.

David Spaven, a rail consultant and Borders Railway campaigner, said the ageing trains should never have been used on the route.

He said: "The 158s have been problematic since they were introduced way back in 1989-90. They've been plagued by problems and they are the least reliable of all the train types operated by ScotRail.

"Transport Scotland has been in charge of this project since 2007/8, on everything from specification of the infrastructure to procurement of the rolling stock, and they really should have gone for something like a Class 170 which operates between Glasgow and Edinburgh. They are much more pleasant to travel in.

"If you go to countries like Germany and Switzerland, when they open new railways they always put new rolling stock in at the same time. Here we're putting in rolling stock that's 25 years old."

ScotRail has invested £14 million to modernise all 40 of its Class 158 trains, which were built under British Rail between 1989 and 1992.

The first of the refurbished units was rolled out to coincide with the Borders Railway opening in September 2015, but they are also used on some Fife, Inverness-Aberdeen, and Central Belt routes.

The upgrade covered new 'Saltire' livery, CCTV, seating, carpets, toilets, lighting, automated passenger-counting technology, and at-seat power sockets, but not new radiators - though the ones currently on board have been updated since the BR era.

The Class 158s were built at a time when BR's budget was severely constrained and their low-cost design has been dubbed "garden shed engineering".

Early problems included the trains' wheels becoming clogged with autumn leaf mulch, preventing the units from interacting properly with the track circuits which detect and alert signallers to the whereabouts of a train on the line.

Later, they were blighted by unreliable air-conditioning systems following the ban on ozone-depleting CFC gases in the 1990s.

A spokeswoman for ScotRail said: “We have identified an issue with Class 158s relating to radiators overheating. We’re just two weeks away from fitting new trial radiators into six Class 158s and will assess their performance in relation to this issue.”

A spokeswoman for Transport Scotland said: “The rolling stock for the timetabled passenger train services are leased, operated and maintained by Abellio under a separate Franchise Agreement with Scottish Ministers.

“ScotRail are required to meet challenging fleet availability and performance targets under the Franchise Agreement.

"Transport Scotland continues to keep performance on Borders Railway under continuous review.

"This is with a view to delivering against a challenging performance target of 92.5 per cent [of trains arriving on time] by April 2019, across the network in Scotland.”