Kerrigan,
The ones you are referring to are made by Atlas and Custom Signals. They use pc boards that can be daisy-chained together with telephone cords and plugs and use and isolated sectional of rail to trigger the signal. Z Stuff signals use an infrared detector to trigger the signal (by a train passing in front of the detector) and only need two wires to hook them up. The signals also can be daisy-chained by means of "extra wires" provided with the signal.
