twelve vs boxcars

twelve and boxcars both are nouns.

Word	Noun	Adjective	Verb	Adverb
twelve	Yes	Yes	No	No
boxcars	Yes	No	No	No

As nouns, boxcars is a hyponym of twelve; that is, boxcars is a word with a more specific, narrower meaning than twelve:

twelve: the cardinal number that is the sum of eleven and one
boxcars: (usually plural) an expression used when two dice are thrown and both come up showing six spots

twelve (noun)	boxcars (noun)
the cardinal number that is the sum of eleven and one	(usually plural) an expression used when two dice are thrown and both come up showing six spots

twelve (adjective)	boxcars (adjective)
denoting a quantity consisting of 12 items or units