I guess I agree with this broadly but I'm not sure the locations trains travel to and capacity is the issues I'd pinpoint.
I've looked into traveling via train a few times now and generally speaking I haven't because the prices were too high and the trains were too slow.
I'd happily get on a train from Maimi to NYC if I could board at 6 pm and get off at 9 am like it really were a hotel on wheels.
But the reality is that it's a 35 hour trip on Amtrack today if there are no delays. I'd have to board at 11 am, get off at 9 pm the next day and transfer halfway.