signalman vs railroad man

signalman and railroad man both are nouns.

Word	Noun	Adjective	Verb	Adverb
signalman	Yes	No	No	No
railroad man	Yes	No	No	No

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

signalman: a railroad employee in charge of signals and point in a railroad yard
railroad man: an employee of a railroad

Other hypernyms of signalman include signaler, signaller, railroader, railway man, railwayman, trainman.

signalman (noun)	railroad man (noun)
a railroad employee in charge of signals and point in a railroad yard	an employee of a railroad