Underground Railroad vs Underground Railway

Underground Railroad and Underground Railway both are nouns.

Word	Noun	Adjective	Verb	Adverb
Underground Railroad	Yes	No	No	No
Underground Railway	Yes	No	No	No

As nouns, Underground Railroad and Underground Railway are synonyms defined as:

Underground Railroad and Underground Railway: secret aid to escaping slaves that was provided by abolitionists in the years before the American Civil War

Underground Railroad (noun)	Underground Railway (noun)
secret aid to escaping slaves that was provided by abolitionists in the years before the American Civil War	secret aid to escaping slaves that was provided by abolitionists in the years before the American Civil War