car train vs railroad train

car train and railroad train both are nouns.

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

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

car train: a train that transports passengers and their automobiles
railroad train: public transport provided by a line of railway cars coupled together and drawn by a locomotive

Other hypernyms of car train include train.

car train (noun)	railroad train (noun)
a train that transports passengers and their automobiles	public transport provided by a line of railway cars coupled together and drawn by a locomotive