I would have to go with the Olympian Hiawatha, during the mid 1950s.
Just to throw out something a little different, I would have loved to have ridden the Camas Prairie Railroad. It was mainly in Northern Idaho and was known as the "railroad on stilts" because of the number of high trestles along the way:
https://is.gd/I7fz6Y
It was a joint venture for both UP and NP.
You can get a glimpse of the tremendous scenery along the route if you watch the movie, "Breakheart Pass", with Charles Bronson (1975). It was filmed on a couple of branches of the Camas Prairie RR. The railroad did offer passenger service, for a while, but low ridership caused the passenger service to be discontinued.
Richard