I just took a sleeper train last night from Helsinki to the arctic circle and they had non-reclining seats with no light dimming. Got around an hours nap between 6am and 7am this morning. Took around 14 hours to go 1000km. Very much regret not paying for a proper cabin…
To counter that, the best sleeper train I ever took was from Beijing to Shanghai, and it felt like I'd travelled to the future (this was in 2008).
Second best might be Portland, OR to Sacramento, though I might have liked it if had been more like travelling to the past (I miss proper dining cars).
European ones have been cheap, cheerful, and uncomfortable, but this was 15 years ago for trips like Florence to Prague, IIRC.
The Zephyr route from San Francisco to Chicago still has a proper dining cart, viewing cart, and is 52 hours long. Surprisingly comfortable and the only way you can access the Ruby Canyon in Colorado outside of a kayak.
6 hours is too short for any special trains (i.e. #001/2 is Moscow - St.Petersburg 8 hour night train) so you can expect that at any route, also attendants have to wake up passengers by a rule.
Added: GP is probably talking about a train that didn't cross a border.