If this is something you're interested in doing more research on, NHK World has a few shows that feature tourist trains. Japan Railway Journal recently had an episode all about them (and many episodes will feature a tourist/sightseeing train after the main segment), and Train Cruise is their monthly show that's basically all about sightseeing via train:
In 2023, Japan Railway Journal showcased 10 tourist trains running throughout the country. Join us as we examine the results of an online poll about these trains.
www3.nhk.or.jp
Criss-cross Japan by rail. Enjoy an amazing diversity of nature and scenery across the four seasons, exploring both well-known and hidden Japan.
www3.nhk.or.jp
I've kind of thought about this too, though in my case I was thinking it might be a neat way to see Shikoku. Not because it has a lot of tourist trains (I think it actually doesn't, though there's at least one big one) but because there are a few JR lines down there that have extremely low ridership and/or stations out in the middle of nowhere (think places that get single-digit daily ridership--HOW) and I kind of want to see what it's like. And it's not like most foreign tourists find themselves in Shikoku unless they specifically set out to get there, as it's generally more accessible by car and there are no shinkansen lines that connect to or go through Shikoku, just one local line that connects to Kansai.