In connection with my recent exploration of the intimate details of the CRJ-200 "regional jet," I also had the opportunity to travel across the entire U.S., from Washington DC to Amtrak's western terminus in Emeryville, California (aka Oakland). My wife and I had done lengthy train trips before, but it had been 20-25 years. Had things changed?
In a word, "no." The train can still be a comfortable way to travel, if you're not in a hurry, don't need the stimulation of television/radio/internet every second, and enjoy scenery. But those are essential conditions. Being seated randomly with others in the dining car; we got great insight into why people were on the train. There were young people (20s) and old (retirees) - though far more of the latter; there were couples, but there were a surprising number of people traveling singly; there were Americans, but there was a large contingent of tourists who undertook a train trip as a way of seeing the U.S. countryside, perhaps hoping to comprehend the vastness of the North American interior.
One demographic was entirely lacking amongst our fellow passengers: There was no one who was on the train because they thought it was the best way to get from one place to another (except for a handful of students we saw, traversing short distances to college).
The passenger train has changed relatively little in the last two or three decades. Passenger accommodations depend of course on what you pay for, but most categories are clean, comfortable, and offer good value for the money. [Possibly to be avoided, if you're not aware of the details beforehand, are the "roomettes," where your daytime setup offers a pair of facing seats, one of which has a handy side table which proves to be ... the lid of your chemical toilet! Many of our fellow passengers seemed surprised by those...]
To continue: Train dining cars serve up tolerably-good-but-not-gourmet food and snack bars have predictably limited, but adequate, choices. Service is generally very good; and the crew seems to take genuine pride in that, not to mention honesty -- no one worries about the fact the cabins can't be locked, and a suitcase can travel all the way across country in the baggage car without being either lost or ransacked. You'll get a lot of reading done, and you'll seldom be bothered by the ringing of a telephone or the need to answer a text, since you'll have little chance to connect to the world electronically except when you're passing through an urban area.
This harkening back to another era is, of course, the main value for many long-distance train riders today. Also key, on the route we took from Chicago to California, is the amazingly beautiful scenery of the Rocky Mountains. It's scenery that can't be duplicated (exactly) by any other means of travel, because the train takes its own route through the mountains in places where the roads don't follow.
Perhaps these things are enough, though it does seem that the numbers of people who want them is certain to decline, making passenger trains even more dinosaur-like. As with other nostalgic experiences, do it while you can.