I have slept up there, and have been relatively comfortable. But there is no way to sit up. I, too, am 6' (but at the time, 250+ lbs), and could not even turn around in my bunk without getting down, or opening the door, or doing something.
Another note about the top bunk is that it is in the curvature of the top of the car - so you get the ... ummm ... feature(?) of enjoying the mass inertia at its furthest point from the pivot point about the wheel trucks. You will feel every nook, bump and curve. I highly recommend putting up the netting. I thought it was to help hold up the bunk, but it's to keep you in it.
You will likely get sleep, but whomever is the lighter of the two sleepers should definitely be on the bottom bunk.
Regrettably, the top bunk on the Superliners don't have windows. When I travel alone, I prefer to sleep on the top bunk and keep the seats below made. I'm pretty ADD, so on a train trip, I find myself bouncing between the seats and the bunk.
The lack of a window for the upper berth in a SL never bothers me. What's to see when I'm sleeping? <_<
As for the ADD --- That's my wife. So my sleeping choice gets me upstairs, out of the way, to sleep. :wacko:
Then she's free to sit up and read, or lay down and sleep, or use the head, or fuss with the stuff in her purse, or do her hair, or walk to the coffee maker, or talk with the car attendant, or go down to the sightseer car, or whatever else strikes her fancy at the moment. :lol: In the middle of the night, I want to sleep.