The first diners were real train wagons. When a train wagon was too old, it was often transformed into a cheap restaurant near a train station.
It is usually narrow and elongated and in them you find stainless steel panels, glass blocks, neon signs and formica furniture.
Diners very often stay open 24 hours a day and serve typical American food. They are very popular and people like meeting there.
Here is a link to get pictures and more information about diners. See also an original project to restore old diners.