It seems that every Christmas greets me (a bona fide railroad nut) with at least one question from a relative about the viability of high-speed rail. The answer is always the same: Depends on the population density and length scale (followed by the ability to secure a dedicated right-of-way). What follows are some numbers that show why we're still a long way from high-speed rail in other parts of this country.
Amtrak's Acela Express is the first high-speed rail in the US. It travels on a ROW largely designed by a private railroad in the 1950s. It attains speeds up to 155 mph and connects Washington, DC to Boston via Baltimore, Philadelphia, New York, New Haven, and Providence. Total distance: 308 miles. Total ridership: 3.4 million. Total profit: $178 million. (Note the $178 million profit number includes state subsidies)
The catch is that this train runs through the major population centers of the US: Washington/Baltimore - 9M, Philadelphia - 7M, New York/New Haven - 23M, Boston/Providence - 7.9M. I'm using Combined Statistical Area since the other statistics seem to have more overlap (i.e. double-counting of population). So the Acela runs 308 miles through a population of roughly 47M people (152,000 people per mile) and gets about 3.4 million to ride each year at a profit of $178 million per year.
The next two most-talked-about potential high-speed rail corridors are the Texas Corridor and the California Corridor.
The Texas Corridor is 240 miles and runs between Dallas (6.8M) and Houston (6.1M). That's only about 55,000 people per mile. If the $178M in "profit" (remember those subsidies) were scalable, the project could expect revenues in the ~$65M per year range. With a cost of $10 Billion, the simple payback on this project would be 154 years.
The California Corridor is 800 miles and runs between Sacramento (2.4M) and San Diego (3M) by way of San Francisco (8.1M) and Los Angeles (17.9M). That's only 40,000 people per mile...and you can see where this is going.
For comparison, projects in the private sector generally require 2-5 year simple paybacks. Federal projects want a 10 year simple payback at most. This Texas thing will prove to be a most interesting development. If they're able to make a go of it and things turn out better than I expect, high-speed rail might become a reality! (I'm not holding my breath, though...)