The longest train ride I have been on is the Trans Siberian from Moscow to Vladivostok. 9000KM. The most memorable one as a couple was a birthday trip for my wife on the Coast Starlight from Seattle to LA. We were in a private rail car with only 8 passengers.
The most complicated trip we took was from Wengen in the Swiss Bernese Alps to Monte Carlo. It involved changing trains 5 times at Lauterbrunnen, Interlaken, Basel, Geneva, and Marseille. We booked First Class, and it was very comfortable.