Unicorn HAT HD
Unicorn HAT on top of Raspberry Pi stack. (Yes, there is already an LED failure.)
Cable and Unicorn HAT mount so LED array can be on top next to track and Raspberry and servo HATs can be under the layout.
The 4 voltage dividers connect the 5v0dc interrupt outputs on the IO Pi HAT to 3v3dc GPIO pins on the Raspberry Pi.