Defect detector

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Standard North American installation of a combination Hot box / Dragging Equipment 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. 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 )[clarification needed] 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.

Seaboard Air Line was the first railroad to install talking defect detectors. Beginning in the 1960s, their train crews could hear the results of hotbox and dragging-equipment checks spoken over their radios in the engine cab and in the caboose. Over the years, as the use of this technology accelerated, the rear-end crews were eliminated from most freight trains. For example, computerized, talking detectors allowed crews to interact with the detector using a touch tone function on their radios to recall the latest defect report. This eliminated any need for crews to walk to the detector location to confirm the radio reading.


Beginning in the 1980s many North American Class I railroads began adding radio transmitters and mechanical voices to their defect detectors; as trains passed detectors, the mechanical or pre-recorded voice would sound out on the railroad's main road radio channel 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 or location, track number (if applicable), 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 and train speed are also noted by the mechanical voice. When a problem is detected, the mechanical voice will often start with a long, high pitch alarm tone followed by a description of the problem and the axle position within the train where the problem occurred. Crews can use their touch-tone hand radios to get the detector to repeat error messages. Defect detectors that are equipped with such a mechanical voice are often called talking detectors by railfans.

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 manually transmitting the error message and required action (like slow down, stop at next station or immediately stop).

Today defect detectors are often incorporated in monitoring platforms that are primarily used by railroads to more closely monitor the status of their trains. In countries where rail transport has been liberalised, infrastructure companies use defect detectors to check the quality and status of different train owners. The main concern of the infra owner is protecting their asset and preventing excessive damage. However, studies have been conducted to see whether defect detectors can be used to issue "penalties" to track-unfriendly vehicles (or provide discounts to those operators with track-friendly vehicles).[1]

Talking detectors 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.


Hotbox Detector

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

  • Hotbox or hot bearing detectors

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.

  • Dragging equipment detectors

A column of cones sits across the whole width of the railroad (just like a cross tie) attached to a switch. Anything dragging from the train will hit this cone, thus pushing it back, thus breaking a contact. It then returns to its normal position to prepare for anything else that might be dragging under the train. The detector will register this action and flag it as a defect. Brittle bars are still used elsewhere, but still have to be repaired. Over time, dragging equipment detectors metal flaps need to be replaced because of extensive damage to them.

  • High car or shifted load detectors

Two vertical white pads sit on each side of the train. Anything that is shifting over will be counted as a defect. It is mainly found on railroads where double-stack trains are prevalent, and where low vertical clearances, such as tunnels, are present.

Sensors for the wheel condition monitor
  • Wheel impact or sliding wheel detectors (often placed at the entrances of delicate high speed track like Amtrak's Northeast Corridor)

Wheel sensors along the tracks feel for flat spots on the train's wheels. Any flat wheel that becomes too dangerous to travel on (a big flat spot on the train wheel) will be counted as a defect.

A bridge spans the railroad with two laser beams that shine down on each side of the passing train. Anything that cuts through the beam will be counted as a defect. This sensor may also be integrated into the high car detector.


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