Mass transit in general, and rail-oriented transit in particular, is regarded as one of the logical responses to steadily increasing costs for liquid fuels. At least in the US, building new rail-oriented transit is usually a long drawn-out process. A question that might be worth asking, then, is which parts of the country "have a leg up" on that process; that is, have made a significant start at building out a rail-oriented transit system.
BosWash corridor, which is not surprising. Chicago's famous "L" is also high on the list. The other twelve cities on the list surprised me in one way: eight of them are in the West (defined from the Rocky Mountains to the Pacific coast) and only four of them are in the rest of the country.
The West has a small number of metropolitan areas with significant populations: Seattle, Portland, the San Francisco Bay area, LA/San Diego, Las Vegas, Phoenix, the Front Range of Colorado, and Salt Lake City. All of those, with the exception of Las Vegas and Seattle, are on the list. Seattle fails to make the list because its system has ridership just over 30,000. Las Vegas's monorail is even smaller, and while there have been proposals to expand the system in either monorail or light rail format, none of those have gone very far. Some of the Western systems are being expanded. Denver, for example, has multiple additional lines already under construction that will go into service between 2013 and 2016, that will substantially increase ridership.
The non-West has many more metropolitan areas on the scale of those like Portland or Denver than the West has. Why aren't there large numbers of light-rail systems growing at the same rate as those in the West? There are many possible explanations, but I'll put forward my own: growth in the West has taught those cities a lesson that much of the non-West hasn't learned. From 1990 to 2010, Alabama was a rapidly growing non-Western state; over that same interval, though, it grew at only half the rate of Oregon. Cities in Ohio, which grew at less than half the rate of Alabama over the same 20 years, would be even farther from proper lesson: at some point, more lane-miles aren't the answer.