I can answer this one. A semaphore is a railroad signal with an arm and sometimes a light to indicate whether to stop (red), proceed with caution or prepare to stop at next signal (yellow) or clear signal, proceed at speed (green).
Photo is not mine.
http://farm1.static.flickr.com/21/31563 ... a5.jpg?v=0
http://farm4.static.flickr.com/3219/272 ... 19.jpg?v=0
It works just like a standard block signal does, just that the arm moves along with the glass for the light. There are either two or three glasses on the back of a semaphore signal arm. Red/Green, Red/Yellow/Green, or Yellow/Green.
red40stang, back me up if I'm wrong on any of this..lol[/url]