storm sewer vs drain system

storm sewer and drain system both are nouns.

Word	Noun	Adjective	Verb	Adverb
storm sewer	Yes	No	No	No
drain system	Yes	No	No	No

As nouns, storm sewer and drain system are synonyms defined as:

storm sewer and drain system: a system of watercourses or drains for carrying off excess water

Other synonyms of storm sewer include drainage well, storm drain, stormwater drain.

storm sewer (noun)	drain system (noun)
a system of watercourses or drains for carrying off excess water	a system of watercourses or drains for carrying off excess water