bullet vs bullet train

bullet and bullet train both are nouns.

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

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

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