The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

signalman
    n 1: a railroad employee in charge of signals and point in a
         railroad yard