Neil Robertson, the Scottish-born chief executive of the National Skills Academy for Rail (NSAR), also cast doubt on the wisdom of Scotland launching an “ideologically driven” hydrogen train project when similar schemes were already underway south of the Border.
However, he also told The Scotsman there were “good things” happening in Scotland in rail, such as the forthcoming re-opening of the Levenmouth line in Fife.
News of Robertson’s attack on the SNP taking the country’s main train operator back into public ownership in April came hours before a key union announcement which could end a months-long train drivers’ dispute that was triggered by nationalisation.
Aslef is due to reveal on Monday the results of a members’ ballot on ScotRail’s improved 5 per cent pay offer, which could lead to the restoration of full services after they were cut by one third in May.
The reduction was forced after the union rejected an initial 2.2 per cent offer, which was tied to the public sector pay limits by which ScotRail is now bound.
Most drivers stopped volunteering for overtime on which services depend, especially on Sundays.
Aslef has recommended approval of the offer, which was increased in return for productivity improvements.
Robertson, who has led the London-based rail skills body for seven years, said of ScotRail nationalisation: "The transfer appears to be based on political rather than economic criteria.
"It’s interesting to note that the only area of overall productivity growth has been in private rail operations.”
As an example, he said privately-run train operators had invested in “longer, faster, better” new trains, which had helped to attract more passengers.
Robertson accepted that a nationalised train operator could be just as innovative, “but history says it’s unlikely because it makes it more of a whim of politicians.”
He said: "I’m not saying the politicians get it wrong, it is just a fact of political life that it’s more short term, and short termism is a disaster for investment in kit and people.
“Will [public ownership] translate into long-term plans with long-term resourcing attached to it?
"Historically, it hasn’t.
"The bits of rail that are in the public sector have had the highest levels of uncertainty historically.”
Robertson also questioned why Scotland had embarked on developing its own hydrogen-powered train, in a project involving Scottish Enterprise, the University of St Andrews and Arcola Energy, now Ballard Motive Solutions.
He said: "They would be much better to put their energies into a collaboration, but it’s classic Scottish politics at the moment – if the button wasn’t pressed in Scotland, it doesn’t count, which is just creating cost issues for everybody.”
However, Robertson added: “We’re still at the stage where there’s quite a bit of innovation in this market, so it’s interesting to have another approach taken.
“I’m running a project called Living Lab which is around bringing more and faster innovations into the rail industry, so it would be remiss of me not to encourage a new innovation, even if it’s slightly ideologically driven.”
Aside from the criticisms, Robertson, who was born in Glasgow and educated in Edinburgh, praised several positive rail developments in Scotland.
He said the “good things” happening north of the Border included re-opening lines such as to Levenmouth in Fife after the success of the Borders Railway being restored between Edinburgh and Tweedbank.
Robertson also applauded the “fabulous” productivity in the rate of electrifying routes in Scotland, which have included to Alloa and Dunblane, with work between Edinburgh and the Forth Bridge getting underway last month.
He said: "As an example of how to do that kind of thing, we are showing others.”
Robertson added that a NSAR study to calculate the wider “social value” – such as jobs – of restoring the Levenmouth line was “really forward thinking and positive, and has encouraged others”.
Scottish Conservatives transport spokesperson Graham Simpson said: “Nationalising ScotRail was always about ideology rather than about service.
“I was asking the SNP what their plans for rail were before the transfer and I was clear they didn’t have one.
"That’s become even more obvious since.”
Scottish Labour MSP and chief whip Rhoda Grant said: “It’s all too clear that the Scottish Government, after years of opposing public ownership of ScotRail, have botched its nationalisation.
“Rail is an essential public service that should be publicly owned and run in the public interest.
“It’s high time that the Scottish Government learned from their mistakes, listened to experts and acted."
A spokesperson for Transport Scotland agency said: “Public ownership of ScotRail is a key milestone in our ambitious Programme for Government to support a greener, fairer, more prosperous Scotland.
"It represents an opportunity to deliver a railway which is for the nation, and fully focused on being run for the benefit of its users – customers, staff and stakeholders, as opposed to shareholders.
"ScotRail passenger services are a central plank in our ambitious decarbonisation plans, backed by £5 billion over the course of this Parliament, to support thriving, resilient and diverse communities.
“This includes exploration of the potential for rail to develop battery and hydrogen alternatives.
"Detailed plans will be developed in the medium to longer term, but our commitment is to remove all diesel passenger trains by 2035.
"Along with Scottish Enterprise, we’re supporting a £3.5 million pilot project led by St Andrews University to convert a retired Scotrail train to be powered by hydrogen fuel cell batteries, which is a really exciting piece of work and a great example of collaboration within Scotland.
“We also welcome recognition of our work to restore lines like Levenmouth and others, providing sustainable connectivity for communities across Scotland and attracting people to visit, live, work and invest in those areas.”