freight train vs railroad train

freight train and railroad train both are nouns.

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

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

freight train: a railroad train consisting of freight cars
railroad train: public transport provided by a line of railway cars coupled together and drawn by a locomotive

Other hypernyms of freight train include train.

freight train (noun)	railroad train (noun)
a railroad train consisting of freight cars	public transport provided by a line of railway cars coupled together and drawn by a locomotive