A JOURNEY time of less than 30 minutes on Scotland's busiest inter-city route between Glasgow and Edinburgh would provide a significant boost to both cities.

As always with major infrastructure projects, the key question is: at what cost? Without a definitive answer, the SNP's Infrastructure Secretary Nicola Sturgeon yesterday committed the Scottish Government to building a multi-billion-pound new line between Scotland's two major cities over the next 12 years.

Whether this is merely a positive soundbite or a canny piece of forward planning will depend on how far and how fast the second and third phases of High Speed Rail progress northwards. At present, the UK Government is committed to building a high speed line (HS2) from London to Birmingham and is due to announce by the end of this year how the line will be extended to Manchester and Leeds. That is expected to produce a Y-shaped network, which would then require a decision about how it is to be extended to Scotland.

As Labour's Shadow Infrastructure Secretary Richard Baker pointed out, however, the devil is in the detail. In theory it would be possible to have a west coast link from Manchester to Glasgow or a route that ran north from Leeds to Newcastle before branching to Edinburgh and Glasgow. If, as seems more likely, there were only one high speed line, however, it would be vital to have a new HS link between the two cities.

To gain the full benefit of the high speed line south of the Border, it is essential that HS2 is extended to Scotland. Transport analysts say that only by cutting the journey time between Glasgow or Edinburgh and London to three hours will significant numbers of travellers take the train instead of the plane. This is essential for making an impact on carbon emissions. It is also crucial for business and tourism that Scotland has good links with cities such as Manchester, Birmingham and Leeds. The benefits of a fast journey from London will be lost because the switch from high speeds to conventional track will cause bottle necks and frustration.

The Scottish Government has agreed in principle to funding north of the Border but it is not clear exactly where the £26 billion required to extend a link from Manchester would come from. Studies have suggested a high-speed rail link to Scotland would boost the economy by some £24 billion, making investment worthwhile.

Ms Sturgeon is right to ensure Scotland's infrastructure will be ready to maximise the benefits of high speed rail but considerably more detail is required. Timescales, routes and costs are all in the balance but governments on both sides of the Border must recognise they have much to gain from co-operation and commitment.