There's a lot of talk right now about the capital costs of high-speed rail - the planned Los Angeles-San Francisco line, which would be the model for America, may eventually cost some $98 billion (or about $75 billion in 2010 money) - but for the most part its environmental benefits are taken for granted. Rail transport tends to be greener than car and air travel, so it stands to reason that as high-speed rail attracts people off the roads and runways, net carbon emissions will fall.
Often that comparison overlooks one critical detail: the environmental damage caused by building high-speed rail lines in the first place. Unless high-speed rail travel reduces emissions by more than what it generates during construction, the project may not be worthwhile from an environmental perspective. Indeed, some researchers have their doubts. A recent British study suggests that high-speed construction emissions may be significant enough to call entire projects into question, writes Eric Morris, who described the work a couple years back at the Freakonomics blog: