Excellent point I hadn’t really worked on. Those architectural differences matter.
OTOH, if the USA magically had a full-fledged high speed rail (HSR) system I also expect it’d have full bore TSA security after some wackos had attacked a train someplace. So that train over plane advantage would not last long.
Planes, of course, yes.
I got to spend three months in Germany about 20 years ago, and travelled around a fair bit, mostly by train. It’s such a different experience than air travel in the U.S., which got me thinking about why that is so. It’s all the reason I already posted, and probably dozens of other factors, too.
It’s not a given that passenger rail would work here as well as it does there, but it’s not impossible, either.
After I posted, I decided to read up on the proposed North-South Rail Link, as it seems to be called. It’s been pretty well researched, and the idea isn’t dead. Lots of info I hadn’t considered.
The U.S. put in full-blown TSA security because planes could be hijacked and turned into guided missiles to kill many more people than who’s on the plane. Some infamous incidents of trains overrunning the end posts notwithstanding 
you simply cannot do that with a train. Nor, conversely, can you pull the emergency brake and take your chances jumping overboard if someone runs amok with a gun or knife on an airplane.
Nor did the government put in full-blown security on the Northeast Corridor, the closest thing we have to real HSR, with frequent-ish and fast-ish service and, more the point, stations in prominent locations in New York City and Washington DC just down the hill from the Capitol. National Guardsmen with M16s standing guard at times, but no security checkpoints.