THE west coast main line (WCML) is the busiest mixed-use rail line in Europe as a result of significant increases in both passenger and freight traffic in recent years.

As a vital link between the north and south of Britain, further demand for services is projected.

This will become a serious problem when the high-speed line (HS2) from London, due to reach Birmingham in 2026 and Manchester and Leeds in 2033, meets the northern part of the west coast line because the sudden switch from speeds of 250mph to half that will cause a bottleneck. As The Herald revealed in March, Network Rail planners say either the additional freight trains or additional passenger trains could be accommodated, but not both. Passengers or freight loads forced to add waiting time or transfer to road lose much of the benefit of high-speed rail. In some cases, it could become more advantageous to make the whole journey by road, adding to pressure on the roads network and increasing carbon emissions, making it more difficult to reach environmental targets.

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Transport Minister Keith Brown argues that to avoid HS2 working to Scotland's disadvantage, multi-billion-pound improvements would be needed to the WCML and it would therefore make more sense to build the high-speed line all the way to Scotland.

There is an obvious logic to this (although some upgrading of the existing line will be necessary for the projected increase in traffic over the next 20 years). However, it is not clear where the estimated £26bn required to extend a link from Manchester to both Glasgow and Edinburgh would come from, although the Scottish Government has agreed in principle to funding north of the Border.

Studies suggest a high-speed rail link to Scotland would boost the economy by some £24 billion. A UK Government intent on cutting public spending to reduce the deficit will be wary of accelerating planned infrastructure spending, but it should not be wilfully blind to the opportunity presented by infrastructure projects to revive the economy through construction jobs as well as the business benefits of transforming the current overcrowded rail network with faster connections between Britain's major cities. Transport analysts have identified a journey time of three-and-a-half hours as the point at which travellers switch from plane to train. That makes the connection to Glasgow and Edinburgh vital in reducing carbon emissions. Ensuring freight capacity to central Scotland is also for reducing congestion on the roads.

UK Transport Secretary Justine Greening has committed to agreeing a timetable for extending high-speed rail to Scotland by the end of the current Westminster Parliament in 2015 and the SNP has said it will set out a timeline by 2016. Clarity, co-operation and commitment must be priorities for both; HS2 will not achieve its full potential, unless it is extended to Scotland.