most if not all of the MTH trackside signals use the insulated rail method to trigger the timing/ action. the 3 aspect CPL i use works that way. when first powered, the yellow turns on, then the green after a few seconds. when the train hits the insulated rail and turns on the relay, the red comes on, green off until the last car passes the rail. then the yellow on for the few seconds, then green.
like Matt, i prefer relays-- they are cheaper than the MTH or Lionel ITADS, and more reliable.
