It has bothered me for a long time that I can't properly experience any of my double deck trains and even many single deck trains are limited. The problem lies with not having different heights for passenger zones.

An example of what I mean by multi-deck, basically just multiple different floor heights, but never stacked.

2-deck double-deck, what many people just call double deck, it consist of just 2 decks connected by stairs.
(note however that the cab cars for the same consist have another deck for the cab)

Multi-deck double-deck, what many people call bi-level (which I don't like since it has more than 2 levels), consisting of at least one section where decks are stacked, but typically connected through a third deck at the end (but not always, examples include the new München-Nürnberg Express and the SNCF Z 24500)

I assume multi-deck support would be easier to implement than double deck support, and it would already help a lot even with "multi-deck double-deck trains", but ultimately, it would be cool to support those too. I don't think MTDD trains would require special support, once the other two types are supported, as they are essentially just a combination of the two.