Without a doubt, this is my all-time fav train journey!! I have been on this trip, southbound, 4 times and, northbound, 3 times.
Must mention that they have tour guides in the summer season who talk about the areas that the train is passing through. Be sure to enquire about this free service when you call for reservations. If you haven't made this trip , Do So! You will NOT regret it, EVER!