bullet train vs bullet

bullet train and bullet both are nouns.

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

As nouns, bullet train and bullet are synonyms defined as:

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