From WordNet (r) 3.0 (2006) [wn]:
diner
n 1: a person eating a meal (especially in a restaurant)
2: a passenger car where food is served in transit [syn: {dining
car}, diner, dining compartment, buffet car]
3: a restaurant that resembles a dining car
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Diner \Din"er\, n.
One who dines.
[1913 Webster]