Is the suggestion here to build more subways/rails? The problem is that more rails would not be useful because there is lower density in the US. Denmark is 141 persons/km^2, whereas the U.S. is about 36 person(census data).
Even if you built train lines everywhere outside urban areas, any stop you step off at would still require a 20 minute drive to get to where you are going.
Density follows infrastructure more than infrastructure follows density. Building a station in a place is more likely to spring up new businesses (and residences) around it. It's not a guarantee and there are a lot of variables in play, but the same reason train stations wouldn't remain "20 minute drive to get to where you are going" are basically the same reasons on average most Interstate stops aren't "20 minute drive to get to where you are going" either.
The density map of the US already resembles the US highway map. Many of the few pockets that don't are explainable with old passenger train maps overlaid on top of that.
Just adding passenger train-only routes on top of existing interstate highways, stopping only at existing exits, would go a long way to service a lot of the US population. It would also presumably spur more walkability efforts at a number of those exits if it was also a passenger train stop.
It may surprise you, but the population density of Tennessee is 67/km^2, which sure, is a lot lower than Denmark, but also a lot higher than 36. Most of the east coast has a higher population density than Denmark. Florida has a higher population density than Denmark. When people talk about the low population density of the US, they're averaging in a lot of sparsely populated land west of the Mississippi, which has fuck-all to do with whether rail lines would be useful in the parts of the US where people actually live.
Florida has a high speed rail that runs between Miami and Orlando and has plans to expand. It's called Brightline, and it's one of the few successful passenger rail projects in recent years in the U.S.
Also population per km^2 can be incredibly misleading, because it's an average. One example could have the population far more evenly spread across the land area, and another could have a dense urban core surrounded by farmland or national parks.
Florida has a slightly higher population than Denmark, but as an example, one of it's largest cities, Orlando, has a population density of ~283 people / km2, while Copenhagen has a population density of 6800 people / km2.
I have lived in Florida all my life. It is sadly the definition of sprawl. There is no way to build enough rail that would serve the average Floridian for day to day transport.
Tell that to Sweden with a density of 26 people/km^2 and a functioning rail system. Also why use Denmark of all places, their rail system isn't even that great compared to e.g. France or Germany.
And the population density of Tennessee is much higher than that of Sweden (67 people/km^2). The states with population densities most comparable to Sweden are Mississippi, Arizona, Vermont, Minnesota, and West Virginia.
But the trains connect cities? How is their density relevant? Moreover the density of the Gothenburg metropolitan area (which includes Mölndal, Hisingen) is 256 people/km^2 and the public transport network spans the whole area.
OP wasn't making a shallow excuse. But this rebuttal does lack nuance :)
The overlapping network effects OP mentioned of how modern metropolises were built, combined with modern bureaucracy, make widespread passenger rail an exceedingly difficult political and economic proposal - far more difficult than it would have been 100 or even 50 years ago.
I'm sure others have mentioned this here, but look at how high-speed rail in California is faring for a good example of the political difficulty involved. The legal hurdles, environmental reports, back-and-forth bargaining between varying levels of government, the NGOs that need to be called in and paid a lot... it's not at all easy. And that's before you get to the structural mismatch between widespread rail transport and all the network effects that led to car culture.
Perhaps Tennessee can fare better due to it being a comparatively very conservative state (which currently codes as having less regulatory burden). But I'm not holding out hope.
