Relocated two IR sensors. In the beginning I realized that there was not room for a CCW semaphore signal on the large oval and located sensors to compensate. That was a bad idea. Better answer is to forget the signals on the outside and put the sensors at the far ends of the oval. Besides the final 10 semaphores are still on back order.
Not wanting to dig into the rat's nest of sensor cables around the Arduino it occurred to me the easy answer. Just change the tag labels on the cables and reassign which Arduino IO pin is paired with which sensor. This fix will also remove the need for "extension cords" around the "X".
Found a Raspberry Pi HAT using I2C, with 32 GPIO pins. This could eliminate the second Raspberry Pi (3A). Ordered one. Pricey.
IF I add the IO Pi Plus HAT to Raspberry Pi (3) I will need to redesign the fan board. The DSI ribbon cable connects under where the parts are currently. There is room across the other edge...
Added a "Roadmap" to the Introduction. A guide to the documentation.
Due to sunlight I can't run the trains except at night or on cloudy days. Shades are in my future.
Replaced back order for double-headed semaphores. Just ordered 8, dropping the two I know won't fit.