Underground Railway vs Underground Railroad

Underground Railway and Underground Railroad both are nouns.

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

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

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

Underground Railway (noun)	Underground Railroad (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