The Collaborative International Dictionary of English v.0.48:

Diner \Din"er\, n.
   One who dines.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

151 Moby Thesaurus words for "diner":
   Brillat-Savarin, Lucullus, Pullman, Pullman car, TV dinner,
   alfresco meal, automat, baggage car, barbecue, beanery, bistro,
   board-and-roomer, boarder, bon vivant, boxcar, breakfast, brunch,
   buffet, buffet supper, buvette, caboose, cafe, cafeteria, cannibal,
   canteen, cantina, car, carnivore, carriage, chair car, chophouse,
   chuck wagon, clambake, coach, coal car, coffee break, coffee shop,
   coffeehouse, coffeeroom, connoisseur of food, consumer, cookhouse,
   cookout, cookshack, cookshop, covered waggon, day coach, diner-out,
   dinghy, dining car, dining hall, dining room, dinner, dog wagon,
   drawing room, drive-in, drive-in restaurant, eater, eater-out,
   eatery, eating house, elevenses, epicure, fast-food chain, feeder,
   fish fry, flat, flatcar, flesh-eater, fruitarian, gastronome,
   glutton, gondola, gourmand, gourmet, grain-eater, graminivore,
   granivore, greasy, grill, grillroom, hamburger stand, hash house,
   hashery, herbivore, high liver, high tea, hot luncheon,
   hot-dog stand, hungry mouth, kitchen, lactovegetarian, local,
   luggage van, lunch, lunch counter, lunch wagon, luncheon,
   luncheonette, luncher, lunchroom, mail car, mail van, man-eater,
   mash, meat breakfast, meat-eater, mess hall, mouth, omnivore,
   omophagist, palace car, pantophagist, parlor car, passenger car,
   petit dejeuner, phytophage, picnic, picnicker, pizzeria,
   plant-eater, predacean, quick-lunch counter, railway car, reefer,
   refrigerator car, restaurant, roomette, sleeper, smoker,
   smoking car, smorgasbord, snack bar, stockcar, supper, tank,
   tavern, tea, tea break, tearoom, teatime, tender, tiffin,
   trattoria, trencherman, truck, van, vegetarian, waggon,
   wiener roast, wienie roast