You have a good point. A lot of the LD trains are really several locals, overlapped by more than one long-distance route, maybe connected by long stretches of almost nothing and almost no passengers that are purely LD. CZ, for example, is:
1. a pretty fast midafternoon to morning 1000 mile LD train from Chicago to Denver -- faster than I think any sane person would make the drive, with a lot of people
2. a very slow scenic train from Denver to Grand Jct.
3. A kind of slow LD train from Denver to SLC and points west.
4. A regional express Denver to Grand Jct. A regional local to Green River, Price, Provo, SLC.
5. An Illinois-Iowa local (or regional express, really, compared to the real locals on the NEC, Downeaster, Vermonter, San Diegan to some extent, etc.)
6. An LD night train across the desert west of SLC.
7. A regional express in California.
And so on.
When I lived in Champaign the City of NO would get dozens of passengers from there to Chicago. Now sometimes we drive to Effingham or Mattoon and use the City in direction or the other; plenty of passengers from those stations north, too. (I loved to take that ride alone and get a nice diner breakfast, but it could be very hard to find seats for a family at 6 am.)
Amtrak seems to make this point politically. I don't know if they advertise strategically (so that the Empire Builder would pop up when anyone along its route searched for Glacier National Park, etc.) Something works, because a lot of people do board not just at the big intermediates (Denver, Salt Lake, KC, etc) but also at quite small towns (Hastings, Ottumwa, etc.) I wonder if they ever study the LDs to see where the stretches with the fewest passengers are, and then try to market those stretches. Everyone gets off in Central Illinois? Offer low fares from there to Memphis, or wherever folks tend to board. Make it dynamic: every time you sell a ticket from Chi to Chm, add a ticket to the cheap bucket for Chm-Memphis, or whatever. I'm not sure how the ads would work, though. There are so few routes and stations, and the routes are so long east-west with no connections north-south, that they couldn't just sell national ads that said "traveling between two little towns? try us!" because most trips wouldn't work. And on the other hand any given station town probably doesn't provide enough passengers to pay to create and publish specific advertising.
Good thought.
I have the image of people finding the train convenient for trips between places that are each a couple of hours from an airport. The western LDs go through a lot of places that can't possibly have very frequent air service nearby. I don't have any statistics to know, but I'd think that the train would be attractive there -- to the relatively few peole wanting to go between two such places.