car vs guard's van

car and guard's van both are nouns.

Word	Noun	Adjective	Verb	Adverb
car	Yes	No	No	No
guard's van	Yes	No	No	No

As nouns, guard's van is a hyponym of car; that is, guard's van is a word with a more specific, narrower meaning than car:

car (noun)	guard's van (noun)
a motor vehicle with four wheels; usually propelled by an internal combustion engine	the car on a train that is occupied by the guard
a wheeled vehicle adapted to the rails of railroad
where passengers ride up and down
the compartment that is suspended from an airship and that carries personnel and the cargo and the power plant
a conveyance for passengers or freight on a cable railway