I am mildly bemused at the lengthy discussion on this thread about the quality of breakfasts and service. The 'hotel on wheels' idea, then becomes the whole issue of quality.
I would far rather prefer a reliable, reasonably priced sleeper service (as alternatives to air), which is not social class/tourist orientated.
The Czech sleeper between Prague and Budapest, I used last year, offered: very good cabin accommodation, key cards, air conditioning, showers at the end of the coach, a quiet layover of two or three hours during the night in a siding to arrive not to early, and an inclusive and adequate breakfast box ( akin to an inflight box of juice, rolls, butter, cheese, liver paste, and jam). No supplements, china plates or attendants having to attend to the whims of passengers, or the accoutrements of class snobs!
This was at a reasonable price of about 40 euros.