So with the Green New Deal on the street there is considerable talk about improving railroad travel. Eli has a couple of perhaps odd points to make.
First high density is NOT something that is needed for fast trains, as a matter of fact it is a hinderance. It doesn't matter how fast a train is if it has to make a lot of stops and every stop includes significant time decelerating as well as accelerating. Moreover close to stations a high speed train travels slowly over normal tracks into the station rather than over a high speed right of way.
Ideally the time between stations should be of the order of one to two hours. For high speed trains this is somewhere between 200 and 400 km (in disgraced units between 120 and 240 miles). Thus, the East Coast Corridor between Boston and DC might not be a very good place to start as can be seen from the marginal reduction of travel time between the faster trains (Acela Express) and the normal ones on the NY - DC route, much of which is due to additional stops for the slower trains.
So where would be a good place to start. There are a couple which suggest themselves. Eli might point to a route linking Minneapolis, Madison, Milwaukee, Chicago, Detroit, Cleveland, Pittsburgh and Philadelphia.
On Twitter, Paul Farrar, pointed out that Texas might be a good place to start. Houston to Dallas has been suggested, but such a route could be built out to include Oklahoma City, Kansas City, St, Louis and Chicago from Dallas, and to New Orleans and Austin from Houston.
So there are many possibilities for high speed rail in the US given the political will.
Which brings the blog to Eli's second point. Railroads in the US are not electrified. Even for freight there are places where considerable gains in decreased emissions and efficiency could be made by electrification of freight routes. One of the bedevilments limiting high speed rail passenger travel in the US has been that both freight and passenger trains travel on the same route. Eli would propose that the lowest hanging fruit is electrification of the major freight routes, followed by building out a separate really high speed passenger rail network, starting in the middle of the country, where the land is mostly flat and population density outside of major cities is low.