I really want to do either of these trips:
https://www.seat61.com/trains-and-routes/bernina-express.htm
https://www.seat61.com/trains-and-routes/glacier-express.htm
I can't decide if Winter or Summer would be better. There are some youtube videos too that show the views. I think the Bernina trip has a corkscrew going up
Man in Seat 61 is a great site for train travel. He goues into great detail on routes & how to buy tickets & transfers
We did both of those routes (also the Gotthard Panorama express and part of the Golden Pass) in September of 2019 during a 3 week trip to Switzerland and western Austria. The Bernina Express is absolutely spectacular and was one of the highlights for me.
The weather was beautiful in September with very little rain, and since it was shoulder season I gather it wasn't as crowded as it usually is in high summer or in winter, though there were still plenty of people around.