> I'm not a fan of the fact that ÖBB couchette compartments don't have locks on their doors

The ones I know have both a turning lock that can be opened from the outside by the conductor or anyone else with a wrench (so more of a protection against someone mistakenly stumbling into your compartment in the middle of the night), as well as a deadbolt that can't be opened from the outside without ripping it apart.