Remember Boris Johnson’s bridge to Ireland? The last PM but one said and did so many preposterously stupid things that it is maybe easy to let this one slip from the memory.

But, yes, “Boris” really talked of a fixed link across or under the terrifyingly deep trench, or Sheuch, that separates Scotland from Ulster.

Everything about the “Irish Sea Bridge” was entirely irredeemably cretinous, starting with its strikingly dumb name. The crossing, after all, was not mooted for the Irish Sea, but the Straits of Moyle, aka the North Channel.

Mr Johnson announced he was an “enthusiast” for the project in 2019. By the fall of 2021 the scheme was quietly dropped. A UK Government feasibility study had found the cost of bridging the stretch of water to be more than £330 billion. For context, that is the equivalent of the entire Scottish Government’s capital spending budget for more than half a century. True, a tunnel would be cheaper, a smidgeon over £200 billion.

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Why so expensive? Well, as most readers will know, it is mostly because the water is so very deep. The bottom of the main trench, called Beaufort’s Dyke on posher charts, is more than 300 metres below the surface. That means you would only see the very tip of the Eiffel Tower if you plopped the Paris landmark in the sea.

Now people – and by people I mostly mean Belfast councillors with heavy chains around their necks – have been fantasising about a crossing between Scotland and Ireland for a century.

This was very handy for journalists like me trying to avoid undermining our transport stories by quoting the kind of dafties who would vocally support hare-brained schemes like the “Irish Sea Bridge”

I am probably giving away a little reporter secret: it is quite helpful when idiots reveal their idiocy by supporting something obviously idiotic.

And it was quite entertaining seeing poor Scottish Tories have to pretend to take Mr Johnson seriously when he banged on about his link to Ireland.

The former PM, of course, started off his career as a reporter: famously getting fired for making up quotes, one of the deadly sins of journalism.

Max Hastings, Mr Johnson’s old editor, once famously said his former staffer “would not recognise truth, whether about his private or political life, if confronted by it in an identity parade”.

I will defer to Mr Hastings on Mr Johnson’s character. But I would like to add this: the former Tory leader always reminded me of what in my trade we sometimes call a “top line” reporter. What do I mean by this? That, as Mr Johnson was the kind of hack, political and journalistic, who had a knack for identifying a headline-grabbing tale. Whether his stories stood up to scrutiny was another matter. But they did sometimes resonate.

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And I think that is particularly true for his bridge to Ireland. It was a magnificently dumb idea. But it had an appealing premise for many: a drive to bolt the disparate and perhaps even fragmenting parts of the British state together with hard infrastructure.

Sure, the “Celtic Crossing”, as the scheme was also sometimes dubbed, was maybe a bridge too far. But Mr Johnson's instinctive desire for "Union connectivity" was not without merit.

I doubt I was the only person who thought about the bridge to Ireland when Mr Johnson’s successor’s successor, Rishi Sunak, last month scrapped HS2 from Birmingham to Manchester.

The British Government has sunk vast amounts of silver in to the link between London and the West Midlands. Its return will now be poor, mostly because the big economic and environmental benefits came from pushing the route further north.

The HS2 network – including legs to both Manchester and Leeds – was budgeted at nearly £33 billion in 2012.

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Officials are coy on details but even without its branch to Yorkshire the scheme was apparently looking like it was heading for a price tag of £100 billion. And that was before anybody bought any actual trains.

Don’t get me wrong, none of this is anywhere near as expensive as bridging the Sheuch. But every kilometre of track between London and Brum is costing the best part of a quarter billion. Or to use another unit of measurement, more than the cost of the two unfinished and endlessly discussed Scottish ferries at Port Glasgow.

Britain has been spending eight – yes, eight – times more per klick on its high-speed rail than France did, for example, on its new route to Bordeaux. Which should give everybody involved a lifetime beamer.

I guess the screw-up of HS2, rather like the problems Scotland has had bashing together new chugga-chugga boats for CalMac, feeds in to a narrative that we live in a state incapable of engineering anything at a reasonable price.

Which, of course, raises even more doubts about whether the UK would have had the wherewithal to go through with Mr Johnson’s North Channel crossing.

After all, if we can’t lay high-speed track over land, what possible chance would we have of doing so over water? The bridge to Ireland would have been one of the most challenging civil engineering projects in the world. Getting bullet trains to Glasgow would be, by European standards, a routine job.

Why did officials not have a grip on Hs2 costs? Well, pass. Smarter people than me will be autopsying the UK’s latest multi-billion-pound transport infrastructure snafu. But this was not just an engineering failure, it was a geo-strategic one.At the risk as sounding as “top line” as Mr Johnson, the Tories had an opportunity to physically and symbolically attach Scotland to the rest of Britain. And they botched it.

Ahead of 2014 indyref there were warnings voting Yes would mean no high-speed rail link to London – and beyond to the continent. It turns out our collective “No” to independence had the same effect. Now a fast train to England feels as distant a prospect as a bridge to Ireland.