For most of the land area of the US, indeed population density is probably too low to support any meaningful public transit, at least for commuters.  I tend to think that public transit in the city with spokes leading out to major suburbs (with park-and-ride lots in the suburbs to account for the sparsity you mention) should be applied more widely.  I've only been to New York a couple times, but I'm a big fan of this model there.<p>What are your thoughts on high-speed rail between major cities?