I took the Toronto to Vancouver train in 1984, and although I understand the details of that route have changed quite a bit since, it was a wonderful experience.
95% of the trip is through wilderness, with occasional stops in small cities that seem to just spring up in the middle of nowhere (Winnipeg...). Once past Thunder Bay, the train suddenly empties out a bit, so it doesn't feel crowded, and spending all day reading, in comfortable seats, with wilderness streaming by the huge window next to you is just wonderful. I found the other passengers quite interesting—I shared my four-seat "section" with a forestry student off to work in a lumbercamp for the summer, and learned a lot of interesting stuff about Canada, and forests... :]—and the vibe was very amiable and quiet. People were friendly, but there was a distinct lack of "chatterers" and most people seemed content to do their own thing most of the time.
Because the trip took five days, and there was basically no demand on my time, it felt incredibly unrushed, and combined with constant low-level "passive" distraction in the form of scenery, the wonderful light, and the soothing motion and sounds of rail travel, it left me feeling both incredibly relaxed and oddly energetic.
Extreme long-distance train travel is generally way underrated, I think; obviously it's not ideal if you're in a hurry, but if you have a few days, it's often a treat. Although the Canadian trip was the best, I've also traveled by train from Seattle to Boston, and it was a great trip too, for many of the same reasons (Amtrak long-distance seats are just amazingly huge and comfortable though).
I was also well prepared for some isolation, with a big stack of books to read, pads of paper etc for writing on etc.
In a way I also think it was easier alone. Travelling with a friend can great fun, but to some degree it can also detract from the feeling of freedom... Because there were some friendly passengers, I never felt lonely, but I also felt completely free to stare out the window and think for hours on end. No pressure.