big dipper vs overhead railway

big dipper and overhead railway both are nouns.

Word	Noun	Adjective	Verb	Adverb
big dipper	Yes	No	No	No
overhead railway	Yes	No	No	No

As nouns, overhead railway is a hypernym of big dipper; that is, overhead railway is a word with a broader meaning than big dipper:

big dipper: elevated railway in an amusement park (usually with sharp curves and steep inclines)
overhead railway: a railway that is powered by electricity and that runs on a track that is raised above the street level

Other hypernyms of big dipper include el, elevated, elevated railroad, elevated railway, ride.

big dipper (noun)	overhead railway (noun)
elevated railway in an amusement park (usually with sharp curves and steep inclines)	a railway that is powered by electricity and that runs on a track that is raised above the street level