In thinking about this some more I like Stan’s idea about mounting only the HC-SR04 heads at track level, maybe in a small 3D printed trackside cabinet, and having the board mounted under table so it’s out of the way. I am looking forward to getting the kits and playing around just to see what we can do with them.
I have no real complaints with the way my current signals, gates, etc operate using mostly Z-Stuff sensors and 153IR’s. But from what is being described in this thread these detectors should be more reliable, not prone to differences in lighting or different paint color problems, triggered by any kind of surface, and easily adjusted to small or wide ranges of sensing. The real test will be tank cars, which IR sensors just mostly do not detect in my experience. 
Rod
