"one who dines, 1815," agent noun from dine. Meaning "railway car for eating" is 1890, American English; of restaurants built to resemble dining cars (or in some cases actual converted dining cars) from 1935. The Diner's Club credit card system dates from 1952.
1. The new diner needs another short - order cook.
2. When asked what a £40.35 meal for five people would cost each diner, they were unable to work it out.