A different line

In practice, there are a number of reasons to be more cautious about the likely cost-effectiveness of a north-south high-speed rail link as a policy for reducing carbon emissions.

Rail's energy consumption — and therefore carbon emissions per kilometre — increases with speed. This means that high-speed rail has higher carbon emissions per passenger/km than conventional rail, and a smaller carbon advantage over air travel.

So there could be carbon costs from passengers switching from conventional to high-speed rail, both because of the energy consumption of the respective modes and because load factors could be reduced on existing rail services; and there would be significant carbon and other environmental costs in the construction of the line.

The impact of high-speed rail on the air sector depends upon how competitive it is compared to flying, in terms of both cost and travel time. The Atkins High-Speed Line study anticipated a nine per cent reduction in air demand for the London to Edinburgh journey in 2016, rising to 24 per cent in 2031.