railroad flat vs apartment

railroad flat and apartment both are nouns.

Word	Noun	Adjective	Verb	Adverb
railroad flat	Yes	No	No	No
apartment	Yes	No	No	No

As nouns, apartment is a hypernym of railroad flat; that is, apartment is a word with a broader meaning than railroad flat:

railroad flat: an apartment whose rooms are all in a line with doors between them
apartment: a suite of rooms usually on one floor of an apartment house

Other hypernyms of railroad flat include flat.

railroad flat (noun)	apartment (noun)
an apartment whose rooms are all in a line with doors between them	a suite of rooms usually on one floor of an apartment house