For decades America's railroads have sought means and methods to detect trackside and mechanical faults that can impact safe operation of trains. Hot box, dragging equipment, high water, shifted load, slide fences, and other detectors are in common use on our lines. One device for detection of ground movements in landslide and embankment failure areas is a mercury-based switch set to trigger a circuit if it becomes tilted. These mercury switches generally work well under catastrophic failure conditions, however they are prone to false alarms (mainly from amimal or human disturbance) and do not work unless there is significant rotation of the post holding the switch mechanism. Several inches or even feet of ground movement can occur without sufficient tilting to trigger the switches.
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