The California High Speed Rail Authority claims that, by 2030, using 100% renewable energy, the high speed rail network will reduce carbon dioxide emissions by twelve billion pounds. At 43 billion dollars to construct in year of expenditure dollars, that represents a cost of $3.58 per pound or $7,901 per metric ton of emissions, a far higher level than any proposed carbon tax. Planting additional forests, however, may sequester 2.2-9.5 metric tons of CO2 per acre per year. At the article's suggested ranges of $200-2,000 per acre, 21.5-215 million acres of timber could be planted at the same price, representing 47.3 million metric tons at the extreme low end and more than 2 billion metric tons per year at the extreme high end. Simply shooting for the same goal as CAHSRA claims, 5.4 million metric tons per year, at the lowest sequestration rate and high end of costs, require an investment of 2.45 million acres and 5 billion dollars, nearly an order of magnitude less. In any case, this amounts to an absolutely trivial reduction in CO2 emissions.
An even better investment, however, would be in the phaseout of the generation of electricity using coal power while simultaneously pushing mass transit and low or zero emission privately owned vehicles through tax incentives and subsidies. According to the California Air Resources Board, the largest CO2 emitters in California are oil refineries although they are put to shame by coal power plants. The Jim Bridger Power Plant in Wyoming, which has a net summer capacity of 2,117 megawatts is responsible for 15,293,640 metric tons of CO2 annually. A $13 billion contract was signed in 2008 for two new reactors providing 1,100 MWe each in Georgia, a cost of some 850 dollars per metric ton of carbon emissions reduced if it were used to replace a plant such as the Jim Bridger Power Plant. While solar does not have the problems of nuclear waste, it's low generation capacity relative to nameplate capacity is a severe detriment in comparison to nuclear and coal and makes it a far more expensive alternative. Proper site design, safety procedures, and modern passively cooled reactors make the risk of a meltdown negligible. Therefore, if we were use to the same amount of money for the high speed rail system as for replacing coal power generation, we could reduce CO2 emissions by 50 million metric tons per year, a far higher amount than CAHSR.
All this is not to deny that there is some environmental benefit to high speed rail. But high speed rail is not the proper or cost-effective means of combatting pollution, especially not with regards to CO2 emissions. High speed rail backers should be wary of making claims that the system is worth the expense because of its environmental benefits when those benefits, when extrapolated to an actual financial cost, offset only a small fraction of the cost of the system.