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. Defect detectors were one invention which enabled American Railroads to eliminate the caboose at the rear of the train, as well as various station agents stationed along the route to detect unsafe conditions. The use of defect detectors has since spread to other overseas railroads.
As early as the 1940s, automatic defect detectors were installed to improve what was normally done with the human eye by railroad workers. The detectors would transmit their data via wired links to remote read-outs in stations, offices or interlocking towers. If a defect was detected, an alarm would sound and the employee on duty would bring the train to a halt using hand or automatic signals.
To this day some rail lines, mostly passenger routes with a very high traffic density, maintain centralized readout, non-talking detectors. This is due to the large and confusing volume of radio traffic a talking detector would generate. When an error signal is received a dispatcher or operator will contact the train via radio and bring it to a halt, manually transmitting the error message.
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.