Regular defect detectors (hot box / dragging equipment) are generally placed at about ten mile intervals. As noted previously; you can hear these broadcast if you use a scanner programmed to the correct railroad radio frequency.
When my youngest son hired out with NS as a conductor; one of the gadgets they gave him to carry in his grip was a small handheld counter or clicker. If a defect detector identifies a problem at axle number such and such; he uses the clicker to count axles as he walks the train.
Curt
