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.
Some commentators acknowledge the uncertainties around the demand forecasts, but argue that if air travellers were made to pay the full external environmental costs of their journey, this would increase the comparative price advantage of rail travel.
The introduction of some form of carbon pricing for aviation… might increase the cost of a one-way air fare from London to Scotland by around £6, including an assumption for climate change costs of other non-carbon emissions from aviation. A price increase of this magnitude would not be expected to have a significant impact on the competitive position of rail versus air on this route.
Rail's environmental benefit compared to other modes will depend heavily on its load factors. A well-utilised rail service has relatively low carbon emissions per passenger/km. But if services are running at low occupancies, the energy consumption and carbon emissions per person could be high — potentially higher than those of alternative modes for the same journey.
Demand for high-speed rail in the UK would be expected to be relatively low on those sections of the route where new capacity would be provided in the hope of stimulating demand rather than relieving existing capacity constraints. The risk of low demand on some sections of the route would therefore have implications for the environmental benefits of the scheme as well as its commercial feasibility.
Rail's comparable energy efficiency and carbon advantage over air travel decreases as the speed of the train increases. This is because air resistance, or aerodynamic drag, increases exponentially with speed, meaning more energy, and therefore carbon emissions, are required to overcome air resistance at higher speeds. In simple terms... a high-speed train would have higher carbon emissions per passenger than a conventional speed train covering the same distance.
The availability of low-carbon sources of energy would have implications for the balance of carbon emissions across transport modes, and it is likely that low-carbon rail will become feasible before low-carbon aviation. If a high-speed rail line could be constructed and powered by entirely carbon-free energy sources, this would increase the potential carbon saving. But the increased carbon savings would need to be viewed against the increased costs, if any, of sourcing low-carbon sources of energy.
While there may be carbon benefits from passengers moving from air to rail, it is likely that there would be carbon costs from passengers moving from conventional to high-speed rail, both because of higher energy consumption and carbon emissions per passenger/km from high-speed rail, and because of possibility of load factors being reduced on conventional services.
Furthermore, wider environmental implications of a new high-speed rail line may not all be positive. There would be significant landscape costs from building new track, including implications for biodiversity, national parks and national heritage. The route of a high-speed train could potentially pass through the Chilterns and/or the Peak District National Park.
A feasibility study by Atkins for the Strategic Rail Authority noted that high-speed rail does not perform particularly strongly on wider environmental implications 'since a scheme requiring such substantial new infrastructure would inevitably have significant negative landscape, biodiversity and heritage impacts, with relatively small benefits to air quality and noise levels'.
Edited extracts of Rod Eddington's report to the government on the long-term links between transport and the UK's economic growth and stability