https://en.wikipedia.org/wiki/Population_density#/media/File...
(Or if you prefer something more recent, have a look at: http://www.newgeography.com/content/002808-world-urban-areas...)
Pretty obvious which areas are ripe for HSR. Except for the mid-Atlantic corridor and arguably coastal California, nothing in the US really is.
It's the same thing when people go abroad then come back and complain how backwards cell phone service is here. People really don't appreciate how underpopulated the U.S. is relative to the rest of the world. (Conversely, it's also why we could accommodate practically an unlimited # of refugees compared to Europe, if we really cared to.)