So with that being said, only one I can think of is for flight attendants during meal/beverage service on a flight.
I rarely, if ever, travel by rail, but I believe tipping is appropriate for meal service on trains. But maybe I'm wrong.
So are there other situations where tipping is understood to be inappropriate?
(And really, I'm probably just limiting it to the U.S., as I know many European countries, and even Asian ones, tipping is not required.)