Tyro (again!) writes:
even in situations where density is there, we still don't have good transit infrastructure. The I-95 rail corridor is substandard. The DC-metro area's density lends itself to a fuller public transit infrastructure, and Boston remains fixated on planning their new public transit infrastructure around slow moving buses. And these are all places that could support rapid mass-transit. We simply don't have the will to create "showpiece" systems in places where it actually makes sense.
Another funny thing about the metro is that, in the various controversies surrounding some basic renovations, one of the members of the board described the DC metro as "The Cadillac of urban transit systems." This was right on as a description-- a Cadillac was very nice in its hey-day, when little else was available, it is still considered luxurious by the elderly and the poor, it's unreliable, unwieldy, and you can find much better models for what you're looking for that are made in foreign countries.