> This small decline was largely the result of changes to the criteria for defining urban areas implemented by the Census Bureau, including raising the minimum population threshold for qualification from 2,500 to 5,000.
So even a small town of 5,000 people is considered "urban". Again, population density is far more important than "urban" population because a big city of 5 million people produces way more demand for travel than a town of 5,000 even if they're both categorized as "urban". Fast passenger rail has high overhead costs in terms of building high speed rial track, and is mostly viable between large population centers. But even the fastest high speed rail is usually 3-5 times slower than air travel. And in America, rail is powered by fossil fuels anyway so it's not like there's a huge climate win in terms of using passenger rail over Greyhound buses or planes.
What changed was the massive subsidies directed towards private vehicle usage. Those rail connections disappeared because they couldn’t compete with the heavily subsidized highway system on all but the busiest routes. This also means that things can change if we start taxing carbon or require vehicle users to pay for their infrastructure.
Of the remaining 20%, it seems unlikely that they’re evenly distributed in areas which are not suitable for rail travel because American history since the early 1800s followed rail lines (and prior to that, canals) because people followed transportation corridors - it was only with mid-20th century subsidies for car travel that people got so casual about ignoring the energy cost of travel.