Sleeper trains sound fun, but I found it tricky to sleep on. I don't think I can sleep in a bed which suddenly moves back and forth.
Anyway here's the cool video I took of our JR sleeper (Aomori City to Sapporo if I remember correctly, through the Seikan Tunnel the world's deepest tunnel at the time): http://oirase.annexia.org/tmp/VID_20110215_213825.3gp (No idea what a 3GP file is, but it plays in my browser.)
I've only ever taken one soft sleeper before, and it was fine. Hard sleepers are OK for me as well (e.g. from Shanghai to Urumuqi), but I was younger back then.
Anyway here's the cool video I took of our JR sleeper (Aomori City to Sapporo if I remember correctly, through the Seikan Tunnel the world's deepest tunnel at the time): http://oirase.annexia.org/tmp/VID_20110215_213825.3gp (No idea what a 3GP file is, but it plays in my browser.)