The highest population density in terms of population per square mile of land is most definitely east of the Mississippi ...
Never thought about using population density. It might be a better gauge than population.
Please don't let population density define where trains should go, other than maybe by looking at counties instead of states.
Population density is only one measure, and it would surely be a mistake to use only one measure.
For a century or more, rural America has been thinning out, losing population compared to the growing cities and suburbs, and even outright losing population. (Must be depressing to live out there. One study showed that Wisconsin has a dozen or so countries that actually lost population in recent years. In 2016 they all voted heavily for the candidate who best expressed their frustration with life.)
Anyway, what matters for trains is not how many live in the hoots and hollers of West Virginia or the wide empty of the western Great Plains or the ridges and peaks of the Mountain States or the deserts of the Southwest. What matters for trains is the population near the stations. Nevada has many many uninhabited square miles of desert deemed suitable for testing nuclear bombs, but that doesn't mean we can quit worrying about service to Las Vegas or even Reno.
The West has many big cities, but they make difficult routes for trains; they are too far apart, almost isolated. Phoenix is the biggest, and it's only close to Tucson. El Paso and Albuquerque have strong cultural links, and El Paso and San Antonio as well, but they are very far apart, and empty in between. Denver and Salt Lake City, sorry, they are 15 hours apart.
Meanwhile don't look at the Mississippi River as the dividing line. The row of states from Minnesota and North Dakota down to Texas and Louisiana has a good supply of large and medium-sized cities, many in the eastern edge of the Plains States. Look at Twin Cities-St Cloud to Grand Forks-Fargo to Sioux Falls. Or Quad Cities-Iowa City-Des Moines-Omaha-Lincoln, Topeka-Wichita-Tulsa-Oklahoma City-Dallas-Ft Worth-Waco-Temple/Kileen (Fort Hood army base)-Austin-San Antonio. Houston-Beaumont-Lafayette-Baton Rouge/New Orleans. Shreveport and Little Rock and Springfield, MO and Kansas City. Somewhere in there I can see at least one, probably two, and possibly three more LD trains, say, 10 years down the road.
