Originally Posted by
seawolf
So it is the physical constraint at TXL that makes immigration check necessary not because one is connecting in Schengen from/to non-Schengen.
Right. Simply compare it to a transfer at most US airports - no transfer area.
TXL originally was built as an airport for point to point connections, mostly from Weat Germany to Berlin and a few flights from France, the UK and the US and (mostly) charter to a few other destinations.