Because at American distances, airplanes are very time-efficient for moving people. And that's today -- they were even more so before TSA.

Trains would mostly be effective at covering those distances that are currently "puddle jumper" flights. Things which are an hour flight on a slow prop plane, but which take at least twice as long real-time due to airport and security hassles.

I don't think anyone is seriously proposing a US-wide HSR network; the US-wide population density obviously makes that non-viable.

There's definitely parts of the US where the population density could easily sustain HSR, though; California from San Diego–Los Angeles–San Francisco is one, and Boston–New York–Philadelphia–Baltimore–Washington is another (quite how far you could stretch that network west is an interesting question, and I don't know enough about current travel trends to comment, certainly Pittsburgh is close enough that travel time-wise it could beat out aviation), as is Miami–West Palm Beach–Orlando for example.

What is interesting is it is likely the case that the North East is the only place where there's a sufficient population density in a larger general area that crosses state lines.

The problems seem to be frequently the fact that nobody wants a train line near them that doesn't stop near them, whereas current aviation routes simply overfly them, and adding in extra stops completely destroys the business case by making them comparatively slow (v. aviation).

Agree completely with the stops issue. Personally, I think that focusing any amount on intercity trains is wasted effort without first having intracity transport worked out. What's the point of having a fast train between Orlando and Miami if I can't do anything without a car on the other end?

No Florida city has a good public transit story. Hell, in the Tampa Bay area, you can't even get from the Tampa International airport to Pinellas county without two hours of buses, because the cross-bay express buses only run for commuters. (Only on weekdays, only during rush-hour times.) So what's the point of a train that goes from Tampa to Miami if it takes me another two hours to get to Clearwater or St Petersburg? That's half of the four hours already to drive direct!

Without even trying to figure business cases out, I think it should be obvious that going after busy aviation routes should succeed without more intracity transport: there's obviously "enough" public transport for aviation to succeed there (where that "enough" might be the fact there are plenty of car hire agencies at the airport along with one or two bus services!).

Unless the alternate mechanism is going to have the same termini,like as the air routes (as in, at the airport itself), it still likely needs additional local transit, even if the existing network is adequate to support the airport.

Pittsburgh is on the other side of the (eastern) continental divide--the Appalachian Mountains--from NYC. They're not the highest but they'd still cause a lot of difficulties to HSR. I also doubt you'd have enough traffic just to Pittsburgh and NYC-Chicago is too far. Even if you wave your hands and reduce the current travel time by half to about 8-10 hours, very few people will take that rather than fly.

NE Corridor already works well for train obviously. Amtrak has various plans to upgrade service though I don't know the current status of those plans.

Half that to 8–10 hours? It's only ~350 miles or so, and if we're talking HSR, by the standards of the past thirty years is only a couple of hours or so. The other thing worth remembering is that some of the steepest grades on railways in the world are found on HSR, up to 4% compared with 1.5% that would be considered steep by the standards of most railways.

Fair point about the grade (given right-of-ways for dedicated track). However, it's over 700 miles as the crow flies. It's academic in any case. This would be a project in the $10s of billions and isn't going to happen in the foreseeable future.

That is correct. America actually has a remarkable freight rail system. But the vast majority of Americans are not involved directly with freight rail so most people don't think about it.

I think in most of Europe there's a relatively painful 50/50 balance between passenger and freight traffic. The freight trains run mostly at night. The tracks are used really, really hard, which causes maintenance issues.

It's quite hard to do track maintenance well when there's a train passing by every 5-10 minutes, 24/7. Maintenance windows can become really expensive.