sports fan vs railbird

sports fan and railbird both are nouns.

Word	Noun	Adjective	Verb	Adverb
sports fan	Yes	No	No	No
railbird	Yes	No	No	No

As nouns, railbird is a hyponym of sports fan; that is, railbird is a word with a more specific, narrower meaning than sports fan:

sports fan: an enthusiastic devotee of sports
railbird: a fan of racing who watches races from the outer rail of the track

Other hyponyms of sports fan include aficionado.

sports fan (noun)	railbird (noun)
an enthusiastic devotee of sports	a fan of racing who watches races from the outer rail of the track