Source : Webster's Revised Unabridged Dictionary (1913)

Signalman \Sig"nal*man\, n.; pl. {-men}.
   A man whose business is to manage or display signals;
   especially, one employed in setting the signals by which
   railroad trains are run or warned.

Source : WordNet®

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