sewer line vs sewer main

sewer line and sewer main both are nouns.

Word	Noun	Adjective	Verb	Adverb
sewer line	Yes	No	No	No
sewer main	Yes	No	No	No

As nouns, sewer line and sewer main are synonyms defined as:

sewer line (noun)	sewer main (noun)
a main in a sewage system	a main in a sewage system