BottleTheWater
Member
You don't need seperate units, they've taken intra-Schengen passengers on Eurostars before by segregating off those carriages from the rest, and sweeping them before allowing new passengers on at Lille. This is probably the easiest way of serving stations to Germany, given that you're going to need to commandeer the existing paths to do so without sacrificing station calls.
Whenever I have entered Germany in the last few years (whether from Austria, Poland or Belgium) the train has been held for passport checks. Would you regard that as "kleiner Deutscher paranoia"?
Sleepers are the sort of trains where onboard checks are actually practical. Or just make use of the facility at St Pancras that used to service the Disney trains.
The disney trains are processed at Lille on the way back.