Brad Plumer recently posted on how high speed rail is not the most efficient way to cut carbon emissions, something I'd previously noted. Unfortunately, it actually overestimates the cost-effectiveness of CAHSR as a means of reducing CO2 because the Authority has not updated their website to reflect revisions to the plan. The new business plan estimates only a three million short ton annual reduction of CO2 emissions, doubling the cost per pound of CO2 in his estimates to $500 per ton.
Furthermore, I do not trust that number. Applied per rider, at the high end estimate of 24.2 million passengers in 2029, it's about 250 pounds of CO2 reduced per passenger which would equate to an avoidance of 272 miles driven per passenger using EPA estimates if every single rider were to have been diverted from driving. Unfortunately for such estimates, a substantial number would be diverted from air travel and according to the International Civil Aviation Organization only 160 pounds of CO2 would be avoided per passenger in such a case (LAX-SFO, economy class). Induced travel, that is, passengers who would not otherwise be making the journey, might actually be considered carbon-negative thanks to emissions caused by their travel to and from the rail system; certainly they cannot be considered to be the source of an emissions reduction, making the estimates further outlandish.