Cheshire Scot
Established Member
A few years ago, when I was travelling on a Euronight from Münich to Ljubljana, the same train also included NightJet coaches destined for Venice (I think it was Venice), which suggests that collaboration with local railways is a part of how NightJet works
Likewise I've joined a HZ-run EN sleeper in Zagreb, and by the time that I'd woken up and was passing Sargans there was a NJ portion on the train, with an SBB loco on the front.
There are a number of cases where NJ and EN services run combined for part of the route.
These can be found by scrutinising: