bullet vs passenger train

bullet and passenger train both are nouns.

Word	Noun	Adjective	Verb	Adverb
bullet	Yes	No	No	No
passenger train	Yes	No	No	No

As nouns, passenger train is a hypernym of bullet; that is, passenger train is a word with a broader meaning than bullet:

bullet (noun)	passenger train (noun)
(baseball) a pitch thrown with maximum velocity	a train that carries passengers
a projectile that is fired from a gun
a high-speed passenger train