What would a high speed railway do for Scotland? The dream is of hopping onto a train at Euston, seeing the countryside whizz by at speeds of up to 250mph on a new, dedicated line, and arriving at Glasgow or Edinburgh less than three hours later.

Such a transport link could wipe out domestic aviation between London and the central belt, relieve the relative geographical isolation of Scotland's two biggest cities and boost its economy by an estimated £24 billion, according to the Scottish Government.

That, at least, is the dream. And it found a pretty good practical overview in Network Rail's 2009 New Lines study, which claimed that building a line all the way from London to Scotland was the best way to gain the full benefits of a high speed rail network, as it would attract the higher fare-paying passengers on long-distance services. The only fly in the ointment was what Network Rail politely described as an "affordability problem", with a gap of around £13bn between the cost of building a line as far north as Preston and completing it all the way to Scotland. Its solution was to phase delivery, with the southern sections built first and an incremental approach to adding on sections north of Birmingham.

With phase one of the UK Government's high speed two (HS2) scheme due to open between London and Birmingham by 2026 and a second phase extending to Manchester and Leeds by 2033, this approach would probably see Scotland get its part of the infrastructure a few decades later.

Fast forward three years and it is interesting to note what has changed in this debate.

With engineers due to get to work on route options and detailed costings for a cross-border line next year, a much clearer picture is likely to emerge of what it will entail. Both UK and Scottish government have committed to put a timetable in place by 2015 for its delivery.

And the commitment this week by Infrastructure Secretary Nicola Sturgeon to build a new high speed rail route between Edinburgh and Glasgow by 2024 will ramp up political expectation that a "dual build" strategy of starting construction in England and Scotland at the same time is the way to go, rather than an incremental approach starting in the south of England.

Addressing a rail conference in Glasgow last week, Jim Steer, one of the key proponents for UK high speed rail and founder of the Greengauge 21 lobby group, highlighted the importance of a Scotland link to businesses in transport groups in the north of England. This is vitally important if what he called a "northern gap" between the Scottish border and Manchester/Leeds is to be bridged.

As well as improving Anglo-Scottish rail journeys, this would relieve pressure on the rail network in the north of England, provide capacity for more local passenger trains and rail freight, which would cut the number of lorries travelling on our overcrowded motorways. Even more enticing was the idea of combining upgrades to the existing West and East Coast main lines with sections of new track - something that would be considerably cheaper than building a new line all the way from London to Glasgow and Edinburgh.

All these factors could prove vitally important if High Speed Scotland is to make the difficult transition from unattainable pipe dream to a deliverable, affordable project. But it's worth bearing in mind what risks being lost in this process, too.

When I spoke to Sir David Rowlands, the then chair of the government's HS2 delivery company, in 2009, he was clear that a significant danger for Scotland was that too many cities would want to be included in a high speed rail route. The more stations you have, the slower the journey between the beginning and end points. The more freight and local passenger trains you have, the less space there is for those 250mph bullet trains.

Instead of high speed, Sir David feared it would become a "medium speed" route as trains stopped at places like Carlisle which have a relatively low population density. When I asked Mr Steer last week about Carlisle he said it would be "barmy" to build a new line north and south of the city and not include it. So clearly there's a difference of views there.

We can expect these tensions to sharpen over the next three years as fiscal reality starts to put more realistic limits on what sort of high speed railway we can afford to build. It might not be the dream we started with but it is surely a better debate to have.

PS. There's been a lot on high speed rail recently and I'll be moving onto new topics next week. Feel free to send suggestions for future blogs, either by Twitter to @DamienHenderson or email at damien.henderson@theherald.co.uk