For a really long trip, I don't much care, except that boarding in Salt Lake City at 3:30 am is an argument for dropping off a sleeper and coach from Calif and picking up one for points east if there ever was one, to allow reasonable boarding times.
But that's on a multiday trip where many stops have lots of pax so the train can't serve most people at ideal times. I think that as a form of transportation for non foamers, the ideal overnight train is one that leaves sometime after supper and gets in to its final destination by around seven in the morning, but allows occupancy until about eight. No meal service and the beds are down when you board. Bos-Wash (to include Providence-Baltimore at reasonable times, too), for example, or Oslo-Bergen, or I imagine London-Edinburgh are like that. A trip like that is effectively faster and more pleasant than flying in the morning and cheaper than flying the day before and needing a restaurant dinner and a hotel.
There's all the talk and expense of hsr for California; I wonder if Amtrak could break even or better running an overnight train between Oakland and LA timed to get people to meetings and conferences in the morning. Just run with the general flow of the host RR's traffic. That's a really slow route anyway: 12 hours for 390 miles or so, so it's just the right length to board, read a little in bed, sleep well, get up, shower, etc.