I point out something similar when I created a spreadsheet of profitable Amtrak routes and thought about how to make it profitable (quit running long ass lines through red-state areas of low density, basically):

“Some basic geometric facts: Intercity rail in the UK works because of the 63 million people in the country, 53 million of them live in England, an area about 16% smaller than the US state of Georgia, or 30% smaller than Washington, which has less than 7 million; most of the rest live in a small belt of Scotland or a pocket of Wales. There are therefore nearly an order of magnitude more people within a distance of each other that can be traveled by rail in a time competitive with flying.

Only two places in the US offer this kind of aggregate mega-regional density, which is essential to sustain a network of intercity trains at a reasonable level of public subsidy: the North East Corridor, possibly extended west out to Chicago; and the coast of California from San Francisco to San Diego. In other places, individual city pairs could make sense (e.g. Portland – Seattle), but those will always be A-B(-C) lines, not part of the network where you can travel widely.”

(Via Talking Sense About Amtrak – Seattle Transit Blog.)

Also quoted in the above article:

My understanding is that the real reason to run the long-haul trains at taxpayer expense is to touch enough states that most of Congress can feel good about Amtrak in general. The other arguments presented here sound largely rhetorical. Ridership may be rising but it’s a long way from profitable or even a reasonably level of subsidy per passenger. “National rail network” sounds like rhetoric without content. Rail is optimal for particular distances. Europe has lots of great rail services, but still, if you’re going 2000 miles within Europe, and you’re not a tourist or time-rich wanderer, you’re definitely going to fly.

My parents just rode the train from Virginia to Ohio last weekend to visit my twins for their 4th birthday and raved about how vastly superior it was to the hassle of flying.

Basically, you take a density map like this:


Anything with mountains on this image within 500 miles or so should have a high speed rail connection.

The vast empty bits?