Defect detector

A defect detector is a device used on railroads to detect and signal problems in passing trains. The detectors are normally integrated into the tracks and often include sensors to detect several different kinds of problems that could occur.

Beginning in the 1990s many North American Class I railroads began adding radio transmitters and mechanical voices to their defect detectors; as trains passed detectors, the mechanical voice would sound out on the railroad's dispatching frequency that a train has passed and note any defects that were found. Most often, after a train has passed such an equipped detector, the mechanical voice will report the railroad name, milepost, number of axles on the train that passed and the phrase "no defects" to indicate that no problems were detected on the train. Sometimes the location's ambient temperature is also noted by the mechanical voice. When a problem is detected, the mechanical voice will often start with "Stop your train!" followed by a description of the problem and the axle position within the train where the problem occurred. Defect detectors that are equipped with such a mechanical voice are often called talking detectors by railfans.

The sensors installed at defect detector locations can include:

• Hotbox detectors
• Dragging equipment detectors
• Clearance problem or shifted load detectors

Defect detectors are used by railroads to more closely monitor the status of their trains, but they are also used by railfans that are carrying scanners to listen in on the railroad's radio chatter. Railfans are often able to gauge where trains are by listening for the detectors' transmissions; in doing so, the railfans can more precisely predict when a train will pass a specific location to improve their chances of photographing the trains.