This board (and the processor) had been removed from the system,

This header plugs into the 2x20 header of the Raspberry Pi 3B+ (3A) to bring out 26 GPIO pins and ground. 

  • On the left is a single 3-pin header with 2 output GPIO pins and ground to control two LEDs to represent traffic density to and from the laptop (4) and the TrainThing hardware. 
  • The three 3x4 headers are for 12 3-pin plugs to connect Wabbit turnout status outputs to the Raspberry Pi. Each 3-pin connection is 2 input GPIO pins and ground.

Picture of Raspberry Pi 3 custome board headers

Header to connect the 24 turnout sensors and an orange/blue traffic monitor.

schamatic of Raspberry Pi 3 custome board