Maybe watch this video: https://www.youtube.com/watch?v=6lupWYQdDgQ

Looking at states is stupid, because 'states' don't limit where people live.

If you look at regions where people live, the US is brilliant for trains and high speed trains.

Population density of whole region isn't as relevant as you think. The most relevant are metropolitan areas and areas of high traffic corridors. That is where trains and mass transit make the most sense.