The route I remember 100% is Copenhagen to Hamburg on a DB train, tickets always bought online at DB (self print). I have to show the credit card first on Danish soil where they merely look at it and check the last four digits. And then again on German soil where they actually scan the card.
As for the internal German routes, I seem to remember having to show my card but can't swear to it.
On the other hand I have two upcoming train trips: Berlin - Dresden and return.
On those it clearly says that the tickets are only valid for me personally together with the credit card. And that the ticket control will scan the card and the barcode of the tickets.