A FlyerTalk Posting Legend
Join Date: Sep 2009
Location: Minneapolis: DL DM charter 2.3MM
Programs: A3*Gold, SPG Plat, HyattDiamond, MarriottPP, LHW exAccess, ICI, Raffles Amb, NW PE MM, TWA Gold MM
Posts: 102,619
In over 4MM of flying in five continents, I've never been asked to show the credit card, but I don't fly LCCs. I've also never had to show the credit card on a train, but my experience with German trains is probably limited to about 25 trips.
BTW, if some airline or airport agent decides to demand something, having an email that this isn't the airline's policy is unlikely to help much.