WRT the west coast, mostly. It's about as long as Japan, but only about half the population. It's certainly populated enough that it's not justifiable that rail travel is so slow.

Less so for the east coast though. From roughly DC to Boston is decently connected with rail, but is not nearly as direct of a corridor as Japan.

The Northeast Corridor in the USA has great rail service. Its the best way to travel between DC and NYC.

It's true to some degree now. But it wasn't very true -- or expected to be true -- back when train lines were being established. That was during westward expansion.