rapid transit vs public transit

rapid transit and public transit both are nouns.

Word	Noun	Adjective	Verb	Adverb
rapid transit	Yes	No	No	No
public transit	Yes	No	No	No

As nouns, public transit is a hypernym of rapid transit; that is, public transit is a word with a broader meaning than rapid transit:

rapid transit: an urban public transit system using underground or elevated trains
public transit: a public transportation system for moving passengers

rapid transit (noun)	public transit (noun)
an urban public transit system using underground or elevated trains	a public transportation system for moving passengers