So that’s why there are those four hyper-dense rail networks on the coasts

yeah pretty much.

the US’s only problem is that these aren’t properly interconnected?

i mean, if you wanted to take a multi day train ride, i guess so? But going from coast to coast is never really going to be a popular route, although likely to be available on some time frames. You're a lot more likely to see 1-4 hour routes popping up in between cities and population centers, rather than across all of the US, granted im sure longer routes will inevitably follow shorter routes given time.

It's probably going to be more annoying, but if you want to do long distance travel, i.e. coast to coast, it's going to be a better experience flying. More cramped, but much, much faster.

There are also issues with freight priority rail, and just freight rail in general. It's a mess right now.