Defect detector

A defect detector is a device used on railroads to detect axle 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.

Before the advent of automated detectors, it was the responsibility of on-board train crew and track-side workers to visually inspect trains for defects, and then to bring the train to a halt, if a defect were observed. To detect "hotboxes," i.e., overheating bearings, they would look for oil smoke during the day or a red glow at night. As early as the 1940s, automatic defect detectors were installed to improve upon the manual process. Hotboxes could be detected using new infrared light sensors; high and wide loads by wires outlining the clearance envelope, and dragging equipment through "brittle bars" - frangible bars mounted between the rails. The detectors would transmit their data via wired links to remote read-outs in stations, offices or interlocking towers. A stylus-and-cylinder gauge would record a reading for every axle; if a journal were too hot, or if some other defect were detected, the offending axle would register a sharp spike on the graph. An alarm would sound as well, and the employee on duty at that locality would either use manual signals or the signaling system to bring the train to a halt and, if possible, to inform the crew of the approximate location of the problem.

Early line-side defect detectors were typically housed in concrete bungalows, roughly every 10–20 miles (16–32 km). When the train cleared the detector, a fixed white light would be displayed from the detector bungalow, unless a defect were found, in which case, the light would either flash or would not turn on at all. The crew stationed at the rear of the train would observe this light and, if a defect were indicated, stop the train and have the conductor report to the bungalow to examine an (analogue) paper-tape readout.

The first computerized detectors used fixed-display boards. These had a three-character numeric display and a number of indicator lights relating to the nature of defects. A number would be displayed in lights on the board after the train had cleared the detector. The number "000" meant there were no defects; any other number warned of a defect at the corresponding axle. If several were detected, small white lights on the top and bottom could also be displayed (depending which light was on ) to inform the rear-end crews of multiple problems and to indicate the type of defect in each case. Nevertheless, the conductor was still required to go into the bungalow and read specific information about the nature of the defect.

...
Wikipedia