signaller vs signalman

signaller and signalman both are nouns.

Word	Noun	Adjective	Verb	Adverb
signaller	Yes	No	No	No
signalman	Yes	No	No	No

As nouns, signalman is a hyponym of signaller; that is, signalman is a word with a more specific, narrower meaning than signaller:

signaller: someone who communicates by signals
signalman: a railroad employee in charge of signals and point in a railroad yard

Other hyponyms of signaller include bell ringer, ringer, toller, whistler.

signaller (noun)	signalman (noun)
someone who communicates by signals	a railroad employee in charge of signals and point in a railroad yard