I sure wouldn't want to be on Tier 3 of that European job
I actually was assigned and slept in one of those while traveling Europe; and didn't find it to tight of a squeeze; I thought it was very similar to a Superliner bunk, maybe even better; as with the perpendicular (to the train direction) setup means that everyone can look out the window. There is also a "little" cubby up there (going on top of the hallway) that is big enough for my backpacking backpack to fit in.
As for privacy; most people either didn't change (In Europe your rarely doing multipul nights in a train) or went to the bathroom to do so; just like one would do in a coach seat. Which however meant that when 'bedtime' drew near there was a bit of a line for the couple bathrooms (2 I think).
I also shared a night in one of the overnight coach compartments and with the exception of not having a great blanket, had no problems. The seats pulled out and flattened down to form a bed going parallel with the train which didn't make the worst of beds (upright on some airplanes is worse, and in my mind upright in general is worse) however I do see issues with sleeping in the bed formation if all 6 chairs are taken. I & the other guy in my compartment early one set ours up for sleeping, so that people passing would not join.
Final thoughts:
I did enjoy sharing the night with someone, In the morning (in the sleeper) you got breakfast (cold, with a hot beverage) delivered and the lower two bunks (on each side) fold up to form seats (issue I guess if your bottom bunker sleeps in) where we (by then only two people were left in the room, including me) sat and ate breakfast and chatted; a lot like the dining cars.
peter
Edit: About having them over here; I would hazard a guess that if Amtrak were to make them reserved (as they are in EU) and had it as an upgrade. When you went to upgrade to it you there would be a little 'description' that would be there saying that this is an Shared Sleeper Compartment, that you may be assigned some bunk mates. And then gave people to option to upgrade to a regular sleeper if they do not want to share; that would eliminate most of the issues. Personally I think the shared sleeper idea is a great one.