Unusual, but they do exist, or have existed.
I once flew on a Lufthansa flight operated by PrivatAir from Stuttgart to Newark which was all business class in a Boeing 737 BBJ. It was one of the most wonderful flights I have been on. The other was the Newark - Singapore nonstop by Singapore Airlines in its previous incarnation using a special performance Airbus 340-500. That was another out of this world flight for me.
Coming back to railroads, India has dozens of overnight trains that are Sleeper only, with no Coach style sitting accommodation and often they are 20+ cars, all Sleepers. So they do exist and are not really all that odd. Of course there are different classes of Sleepers in those trains.