Defect detectors

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, on-board train crew and track-side workers used to visually inspect trains for defects e.g. "hotboxes" (overheating bearings) would smoke or glow red. By the 1940s, automatic defect detectors included infrared sensors for hotboxes, wires outlining the clearance envelope to detect high and wide loads, and "brittle bars" – frangible bars mounted between the rails – to detect dragging equipment. The detectors would transmit their data via wired links to remote read-outs in stations, offices or interlocking towers, where a stylus-and-cylinder gauge would record a reading for every axle; a defect would register a sharp spike on the graph and an alarm would sound or a visible signal would be given to the train crew.

The first computerized detectors had lights indicating the nature of defect and a numeric display of the associated axle number.

Seaboard Air Line was the first railroad to install defect detectors which "spoke" their results over radios carried by train crew. Later models allowed crews to interact with the detector using a touch tone function on their radios to recall the defect report. Today, defect detectors are typically part of the general monitoring platforms keeping track of train status.

The sensors installed at defect detector locations can include and are explained:

Two infrared eyes sit on each side of the tracks looking up at the train's bearings. They register the radiation from every journal that passes over them. If a bearing reaches the maximum temperature for safe travel, the detector will flag and count it as a defect.

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