It would be SCAX aka Metrolink and that is the defect detector where marcotor says. It reads out if there are any issues, hot boxes; dragging equipment; etc., with the train passing over it.
That is probably it, as I hear it well out in the Inland Empire and south to Moreno Valley. Also can hear it in the San Gabriel Valley. And it broadcasts every train that passes thru, and says "no defects".