NOTE: The Squealer itself is $80 and comes with the base unit, speaker and a sensor. An additional (optional) sensor is $20 dollars and the optional DCC to DC converter is another $10 dollars for a total of $110.
The Squealer is a motion detecting flange wheel squeal system by the good folks at ISE. You can order yours at Iowa Scaled Engineering: https://www.iascaled.com/store/ModelRailroad/Squealer