A diner is a person who's eating a meal, and it's also a word for a casual restaurant. If you pass by a diner in a diner, check out what's on his plate. It could be anything from eggs over easy to salisbury steak.
If you dine in a cafe you're a diner, and if you eat in the dining car on a train, you can call it a diner too. Your favorite neighborhood diner — a casual restaurant that keeps late hours and serves a variety of food — is actually named after these railroad restaurants. The original diners, from the 1930s, were shaped like train cars, often clad in stainless steel, and had long, narrow space inside with stools along a counter.