If you really want to get a "thank you" home and you're prepared to spend some effort finding the details, a letter written to the head of department concerned (rather than just to customer relations) is a surer bet. I did this for a cabin crew member in a Big Airline based in London. Not only did I receive a personal letter in reply from the head of cabin services, I heard independently that the cabin crew member concerned was shown the letter in a friendly and pleasant chat with their line manager.