Most locals and "non-tourists" don't take it end-to-end and the majority never have. Tourists are generally taking it end-to-end (though not everyone does) but most of the passengers at any given moment are non-tourists. It's not a specialized luxury tourist train or something as you seem to be implying. In many ways it's analogous to taking Amtrak coast-to-coast. TS is a great trip with lots of interesting folks to meet and sights to see - and if it isn't a very memorable trip you're doing it wrong!

Anyway it's not clear why folks' most memorable train rides can't be on a particular train - what difference does it make? Or why said trip can't be from when one was younger, which for some was just a few years ago!