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I think I commented when askod posted this earlier that I don't think any simple claims can be made. We are talking about factors here that each have ranges in orders of magnitude:

  • High-speed line electricity supply: it obviously makes a difference whether it comes from coal-fired power plants or hydro, for example.
  • High-speed line construction emissions: with the same materials technology (because most CO2 emissions are from manufacturing steel and cement), it basically depends on the volume of superstructures, and there is a great difference between a line consisting of level track on flat stable ground in a sparsely populated area and a line consisting almost only of viaducts over soft soil and villages.
  • Traffic level: there are high-speed lines carrying 1-2 million passengers a year, and ones carrying 138 million passengers a year (two orders of magnitude!).
  • High-speed line renewal: how long do the different parts of the infrastructure last? (The construction-related CO2 emissions will have to de divided by the total number of passengers over that period.) This is not straightforward. We are speaking of components with a life on the range of decades, and it can both happen that something lasts multiple times longer than originally planned (due to too conservative expectations or advances in maintenance), or that something lasts a fraction of that time (due to bad quality control, unexpected events or corrosive processes, or an upgrade that became a demand or necessity earlier than a regular renewal).
  • Distance: this is a strong factor for air transport CO2 emissions.
  • Airport size and location: air transport related emissions also include some significant construction emissions (runway, terminals, connecting highways and rapid rail) and emissions from airport access transport (which can be anything from a lot of private car driving to a short tram ride).

On the long run, I do think that construction-related CO2 emissions can be reduced significantly. There is research both on the steel and cement front.

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Tue Jul 19th, 2011 at 04:39:13 PM EST
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