Today’s big question in the exciting and fast-moving (DYSWIDT?) world of high-speed trains: is Britain’s proposed HS2 rail link upside down?

That’s certainly the view of the Scottish National Party, who make it clear in today’s manifesto that it should start in, well, Scotland. It’s just possible that they’re a teensy bit biased on this one, but let’s run with it.

Some background here. Being the grandest of grands projets, HS2 is going to be a bit expensive, and consequently it’s going to be built in stages. Stage 1 will link London and Birmingham; stage 2 will extend the line along two branches, an eastern arm to the East Midlands and Yorkshire, and a western one to Liverpool, Manchester and points north west.

Anything that happens after that is so speculative that no one has even bothered to number the phases. But it’s widely assumed that the natural thing to do would eventually be to extend these two lines yet further north, to Glasgow and Edinburgh in the west and Newcastle in the east. In the meantime, you’d get trains using the up to 250 mph HS2 as far as the M62 corridor, then existing infrastructure to complete their journey; in effect, the far north would still get high speed trains.

As far as the SNP are concerned, though, this is all wrong. Over the weekend, deputy leader Stewart Hosie told Sky News that one way his party could influence a future Labour government at Westminster would be start at the other end.

…if there was a minority Labour administration which needed our help in order to get their legislation through, we might well make a case for saying look, let’s not have HS2 go to Manchester and Birmingham, let’s have it start in Edinburgh, Glasgow, coming through Newcastle as well, so we have a joined up high-speed rail network across the whole of the island, to the benefit of everybody, not just those travelling from Birmingham to the South.

(H/t the Birmingham Mail, whose readers may understandably be a little grumpy about some of this.)

The manifesto published this morning gives a softer version of this policy, in which the “top down” high speed rail link seems to be in addition to the “bottom up” one already on the agenda. Here’s the key paragraph:

For a new northern focus – we’ll back budget plans to invest more in the infrastructure of Scotland and the north of England, including the commissioning of high speed rail linking Glasgow, Edinburgh and the north of England.

The political message here is clear: despite what the Tories are saying, English voters need not be scared of a powerful SNP presence at Westminster. That’s an important message, because one of the best arguments against more powers for Scotland is that it already gets a much, much better deal than any other region of the UK that is outside London’s orbit.

Fair enough. But nonetheless, there are questions about this policy. Consider what the new rail link is intended to achieve.

HS2 has two key purposes. The first is to boost regional economies by improving links to fast-growing London. Edinburgh and Glasgow have pretty booming economies, it’s true – but even combined they’re a lot smaller than London. So, if you accept the argument that better transport links will work for the northern cities, it’s not clear that an expensive link to Scotland will have the same impact as an expensive link to London. (Newcastle, which unlike the other northern cities is much closer to the border than to the capital, may be an exception, and probably would benefit from a high speed rail link.)

The other purpose of HS2 is to boost capacity: it’s about the fact that if we don’t build something like this the existing train network simply isn’t going to have enough space for passengers. The biggest pinch point is on the West Coast Main Line into London, which is why the government has already spent a small fortune planning for it.

That makes Hosie’s idea of scrapping the London-Birmingham link entirely a non-starter. The reason the line is starting there is not that London makes all the decisions (at least, not entirely). It’s that this is where the line is most needed. The trains from Edinburgh to the north of England simply aren’t creaking in the same way.

We could build HS2 at both ends; but abandoning the London leg entirely would be a disaster.