I’ve just found a post by Brad Templeton arguing that US mass transit is less green than high-efficiency cars, at least when compared per passenger-km. (He agrees that transit is overall better because it is more efficient when used more extensively, as in Europe and especially East Asia.) The analysis of how this can be given the numbers is cogent, but the numbers themselves are suspect, and are worse for transit than other numbers I’ve seen.
Better numbers can be found in this FTA presentation, on pages 10-11; the data is sourced to the National Transit Database. They’re expressed in pounds of CO2 per passenger-mile; if you’re more used to thinking in terms of passenger-miles per gallon of gasoline equivalent, then convert x pounds per passenger-mile to 19.374/x passenger-miles per gallon. The New York City Subway gets the equivalent of 114 passenger-mpg, versus 47 on Templeton’s page. Even FRA-regulated commuter rail does significantly better than cars – the low efficiency of the trains cancels out with the fact that there’s almost no off-peak traffic.
Another piece of evidence Templeton’s transit numbers are too low: he lists JR East’s energy use as equivalent to about 78 passenger-mpg. In reality, JR East claims much lower emissions, about 13 grams per passenger-km (400 passenger-mpg equivalent) or 19 (280), depending on whether one counts the emissions of the company’s buildings or just transportation emissions. It could be that Japanese power generation is that efficient; but given that Japan’s overall per capita emissions are not low by non-US developed country standards, I doubt it.
Finally, although it appears as if technology is about to make cars much more efficient, in reality technology is expensive if you’re a driver and cheap if you’re a transit agency. Take hybrids: the market share of new hybrid car sales is in the single digits, about 300,000 out of 8 million light vehicles sold in the US in 2008, but the market share of new hybrid bus orders was 22% in 2007. Electrified trains are also gaining efficiency, perhaps more slowly but the important thing for them is to transition to low-carbon power generation; if their emissions are nontrivial thirty years from now, then we have bigger problems than transportation to worry about.