This is an Arduino Uno shield that can be used as a model railroad DCC packet sniffer. The board holds four pluggable terminal blocks to distribute track power, and an optocoupler that enables the Arduino to monitor DCC traffic.
A couple of pins can be added to easily connect oscilloscopes, protocol analyzers, etc.
The circuitry is based on an article by Neil McKechnie found on Neil's Blog.
Using this shield you can get a nice packet sniffer dumping DCC packets to the Arduino USB port using the code found in the DCCInspector-EX Github repository.
For more information, visit the project homepage.
The files provided are project files for KiCad 5.