America's passenger rail is the sad, neglected stepchild of passenger transportation in America. In recent years, there has been growing support to reinvest in trains, especially high-speed ones, as a cheaper and less polluting mode of intercity transportation than driving or flying. The constant argument against more investment is simple: no one takes the train, so why pour more money into it? But perhaps no one takes the train because there are no trains to take? Look at this little animation (from Greater Greater Washington, via Grist), that shows the creeping death that budget cuts have brought to Amtrak service:
As you can see, service is robust in the '60s, covering nearly the entire country—that's before Amtrak even started. In 1971, with many of those lines going bankrupt, Amtrak took over with a much more limited portfolio of routes. Even those limited routes have been cut ever since, except for a little recent uptick. It's like watching a sick person slowly wasting away.
The economics of riding the train are different now than they were when the advent of the passenger car—and then short-haul flights between cities—started eating away at rail travel's market share. It's now grossly inconvenient and annoying to board an airplane; you can walk on a train with no hassle (though the discovery of Al Qaeda plans to target American passenger trains might change that). And driving no longer has quite the same open-road sense of freedom; there are no traffic jams on rail lines. Though technically, that's not quite true. In many parts of the country, passenger trains lose the right of way to freight trains, which can cause them to wait interminably. But that's just another example of how we've diminished rail travel through a death of a thousand cuts, and if we devoted money and resources to it, it could be a viable form of travel.
If rail travel was cheap, and convenient and fast, there is no telling what the market might be. In the Northeast corridor, where the trains run fast and frequently and between closely situated cities, ridership and profits are up. Now imagine if the trains went 300 miles per hour. Then you might see crowded trains everywhere.