I mentioned it in a previous post related to the Milwaukee Road in the West, but I also think that July 1974 trip on the "North Coast Hiawatha" was my favorite passenger train ride. I rode from Chicago to Spokane in an ex-NP dome sleeper; the car was original inside and out, and in excellent condition. There were five sleepers in the train, but the dome (just a few steps from my bedroom) never had more than two or three other passengers in it. There were at least two dome coaches towards the front of the train, and between Chicago and Minneapolis there was an ex-CB&Q parlor/dome/observation on the rear. The weather was beautiful, and the trip across Montana was spectacular. I particularly remember the descent into Butte, and running parallel with the Milwaukee Road along the Clark Fork River.
My other favorites would have to be the California Zephyr Chicago-Oakland in 1968, the Rio Grande Zephyr eastbound in 1971, and the Southern Crescent (still run by the Southern Railway) in 1973, riding in the dome/parlor car between Atlanta and New Orleans.