The question has been asked many times. What, exactly, is the definition of a diner? for my own part, I have often stuck to the party line of 24 hour breakfast, or else I've offered some half-hearted claim that I know a diner when I see one, but to try to describe definite characteristics which make one restaurant a diner while another is not is somewhat difficult, to say the least. However, I'd like to make some attempt here to define (or perhaps interpret) what a diner is.
For one thing, there are a number of characteristics which many diners share. Most are open 24 hours a day, or at least extremely late, and serve breakfast for the whole time. Most serve coffee (black as death) as a staple. Another typical characteristic is one of several themes: Hellenic, Neon '50's, or Abandoned Train Car. Fare ranges from American to Italian to Greek, but any diner will be able to give you cole slaw, pickles, some sorts of pastries, waffles, coffee (this point bears repeating), grilled cheese and so forth. Finally, typical diners have jukeboxes at tables, or somesuch.
But, though these qualities can be found in many diners, there are some which possess few, or perhaps none, of them. It becomes rather tricky to say which of these attributes are strictly necessary to be a diner, and which are merely unexpected treats, turning an otherwise pleasant diner into a great one. Calling oneself a diner is insufficient. The Penn-Can Restaurant and Truckstop, for example, makes no such claim, but in visiting it, one becomes instantly aware that this is a diner of no mean standing. The Ramapo Forum Diner, on the other hand, while certainly a good place to eat, left me with some doubt that it was anything other than a family restaurant. An even more insidious example of a restaurant masquerading as a diner is the Tiffany Diner near Philadelphia. In this case, an early closing family restaurant adopts the title of diner for some evil purpose of which I have no clear understanding.
However, despite the elaborate deceptions set up to beguile and confuse us, I think that a couple of things can be said with reasonable assurance. The first is that diners can be roughly divided into two type: The Suburban, and the Highway. The former is usually a restaurant which is open late (if not 24 hours), which caters to the old folks, or the high school kids, and, late at night, to the worst elements (such as my friends). The latter, however, is what the purists idealize when they talk of diners. They spring up to serve the needs of travelers and truckers; two groups which may need sustenance (and especially coffee) at any hour of day or night, and who can only survive the consumption of fast food for so long.
Some purists, therefore, may consider only the Highway diner to be a true diner, but I have to disagree. While no doubt the call for Suburban diners is different than that of Highway diners, the reason is the same: People need somewhere cheap, bustling, good, and yes, even greasy, where they can meet, eat, talk, and so on. After all, who can go to a true diner and not people watch? A regrettable side effect of all of this is that the line between the Suburban diner and the family restaurant has been blurred, and as such, people sometimes can't tell the difference. In that case, remember: If you are in complete doubt, look for speckled formica, linoleum, stainless steel and moldy wood. Chances are, if you find all of them in the same eating establishment, you've found yourself a genuine diner.