Public Transport

In theory that means that one entire "floor" of the Luna is accessible, compared to just the vestibules on the traditional double-decker.

In most double deckers in Germany you enter on the lower floor between the stairs and the bike space (foldable chairs and rails to strap bike to) and the stairs. Between the bike spaces is a regular sitting area separated by doors. A third up the stairs is the connection to the next cart and sometimes the toilet (some models have a accessible toilet on the ground floor), then the stairs double back for the upper sitting area.

The sleepers trains had cabins with bunk beds, making the lower ones accessible. The proposed sleeper trains have stairs to each "pod" and a very narrow corridor.
If they want to go double decker they should retrofit normal ones with beds, maybe two on top of each other on the bottom floor, and one on the top floor at higher cost.