Here, in the video, you could hear an electro-mechanical signal controller in operation.
It is a Marbelite M-30, and it was once located in Coney Island, New York. Today, a computerized signal controller is what now exists.
The neat "ker-chunk" is produced when the solenoid changes the positions of the cams in the cam assembly. Ultimately, the cams themselves are what actually change the indications of the signals
that are wired to this E/M signal controller.
Lovely sound to my ears.
http://www.youtube.com/watch?v=b5gQav7PI_M
