subway train vs railroad train

subway train and railroad train both are nouns.

Word	Noun	Adjective	Verb	Adverb
subway train	Yes	No	No	No
railroad train	Yes	No	No	No

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

subway train: a train that runs in a subway system
railroad train: public transport provided by a line of railway cars coupled together and drawn by a locomotive

Other hypernyms of subway train include train.

subway train (noun)	railroad train (noun)
a train that runs in a subway system	public transport provided by a line of railway cars coupled together and drawn by a locomotive