I have a similar, but more attainable train goal of riding the Amtrak Empire Builder route from Chicago to Spokane. The route looks beautiful and has a few nice stops along the way.
If you want a shorter version that cuts out all of the flatlands, fly to Great Falls, rent a car and drop it/board at East Glacier and then ride it to Spokane.
I don’t see them shutting down all the long haul routes in the next decade or two. They would have to keep servicing the segments with shorter routes, which probably means they’d have to invest in some infrastructure, and I don’t see them getting the funding for that anytime soon. And even if that happens, they would probably save a couple of long haul routes for historic or cultural reasons. And given how popular the Empire Builder is, and how sparsely populated areas it goes through—meaning it is harder to justify the infrastructure that enables shorter routes—the Empire Builder will probably be the last long haul route to go.
I think the Empire Builder will still be there in 2040, even if it will only be a weekly run, mostly for tourists.
Rode this going the other way (Portland to Chicago) this summer! It’s a gorgeous ride, at least until eastern Montana, which is a whole heck of a lot of nothing for a very long time. Totally worth it for going through the Columbia River Gorge and passing through Glacier National park, though!
Pay attention to the timing of your ride! I rode this trip once and didn't get to the Rockies until it was too dark to see much of anything out the windows.