Railroad defect detector database. There was a train derailment in East Palestine, Ohio, that released carcinogenic vinyl chloride. According to rumor the first people to know there was something wrong were "open source intelligence" (OSINT) people using software defined radio to listen in on defect detectors along the tracks. This website gives you a map of defect detectors and their frequencies. You can zoom in on your area and find the frequencies for defect detectors on rail tracks near you.
Railroad defect detectors do such things as measure the temperature of wheels and notice if wheel bearings are getting too hot, count train cars to make sure they're all there, and detect loose parts or equipment getting dragged on the track. It uses a VHF transmitter that is picked up by the driver up front, but anyone can pick them up. In my area (Denver), I can see 3 Union Pacific defect detectors and 1 BNSF defect detector.
