diner-n; 3 Senses

Sense Number 1: a person eating a meal, often in a restaurant

Commentary: DINER[+person][+eater] [+role][+eater][+meal]

The diner knocked over his wine glass reaching for the salt shaker.
John is a frequent diner at the new Vietnamese restaurant.
Diners will enjoy the beautiful outdoor deck overlooking the sea.
The seagulls were unwelcome diners at our beach picnic. (figurative/ironic)
Two of the diners at Mary's Thanksgiving dinner became ill from the mushroom soup.

WordNet 3.0 Sense Numbers: 1

Sense Number 2: a railcar on a train where food is served and eaten

Commentary: DINER[+vehicle][+car][+train][+eating]

We had a glorious lunch in the diner of a first-class Italian train.
Most American trains no longer have real diners but just simple snack cars.
We could only get reservations for the second seating in the diner car.

WordNet 3.0 Sense Numbers: 2

Sense Number 3: a casual restaurant resembling a dining car on a train

Commentary: DINER[+building][+railcar][+restaurant][+casual]

Bob ordered a hamburger, fries and a large chocolate shake at the local diner.
The waitresses in that diner wear pink uniforms and short white aprons.
It's a fashionable new downtown diner with 50's retro decor.

WordNet 3.0 Sense Numbers: 3