In an article by Robert S. McGonigal, defect detectors along the tracks are explained in detail.
(The article may be found at Defect detectors | Trains Magazine)
An Amtrak train en route from Milwaukee to Chicago on Canadian Pacific’s double-track main line hurtles by a metal cabin and some trackside apparatus. Over the radio, a stilted voice intones “CP detector, milepost five seven point six. Main track: two. Total axles: one six. No defects. Temperature: five three degrees. Detector out.” A moment later, the engineer’s perfunctory acknowledgement: “Amtrak 332, no defects, 57.6.”
Train 332 has just passed a wayside defect detector, one of thousands employed by the railroads to monitor trains and the fixed plant. Most such devices scan passing trains for defects like hot bearings, hot wheels, dragging equipment, and high, wide, or shifted loads. After inspecting a train, modern wayside detectors will automatically report their findings by radio. Other installations keep tabs on the right of way, watching for environmental conditions that pose hazards to trains.
Hot bearing detectors
Overheated wheel bearings (journals) were once the scourge of railroading. Journal boxes contained lubricating material that needed frequent repacking; overheating was common, resulting in “hotboxes.”
Undetected, a hotbox could turn into a burned-off bearing – and a derailment. Rear-end crews were always alert for the telltale smoke and smell of hotboxes in the train ahead of their caboose.
Today, plain-bearing cars are all but gone, but the successor roller bearing is still prone to occasional overheating. Likewise, hot wheels, often caused by sticking brakes, have long been a hazard.
The mid-1950’s saw the introduction of infra-red wayside hotbox detectors, which monitor the heat profile (compensated for air temperature) of each of the train’s bearings, even if it passes at high speed.
A typical installation consists of two units, one outside each rail, aimed upward. Such devices may also detect hot wheels, but single units aimed across the track (diagonally, to catch both sides), at mid-wheel level, are also in use. On-board monitoring systems are being developed.
Dragging equipment detector
Dragging equipment can damage the track and grade crossings, is often a sign of damage to the train, and can cause a derailment. In fact, dragging equipment can be a derailment; one or two wheelsets can be off the track but still travel for many miles before piling up the train in a wreck.
Early dragging equipment detectors (or “draggers”) were of the “brittle bar” type. Fixed elements between and beside the rails would break when struck by foreign objects. Their breakage would interrupt an electric circuit that formed part of the reporting system, and the train would be stopped and inspected.
The introduction of “self-restoring” draggers in the 1960’s, which are hinged and sprung so they return to position after impact, reduced maintenance requirements.
Modern detector installations
Hot bearing/hot wheel detectors and dragging equipment detectors are often part of the same installation.
Detector spacing depends on each railroad’s preference and local conditions, but 20-30 miles is a typical interval on main lines. Employee timetables list the locations of hot bearing and dragging equipment detectors (abbreviated HBD and DED, respectively), along with their data-reporting methods.
Early detectors were linked to the wayside signal system, so as to show a red signal to a train with a defect. Others reported their findings to the dispatcher, who could have the train stopped if a defect were found. Later types used a scoreboard-style sign to report the train’s condition to the caboose crew. On some others, the display of a simple illuminated light indicated a train had no defects.
Modern detectors are of the “talking” type, which use a prerecorded voice to report a train’s status over the radio, although some visual reporting methods are also in use.
Detector radio protocol varies. Communication can be limited to the detector’s identification of itself and the statement “no defects.” Others will first announce that they are working, then give a report after the train has passed. Some, like CP’s at milepost 57.6, give the ambient temperature as a check; others report the train’s length in feet. Some railroads require an acknowledgement from the crew of passing trains; others do not.
If a detector finds something, it will announce “you have a defect” or “stop your train,” then the location of the problem. The crew makes an inspection from the ground, paying special attention to the cars at and behind the cited location, as most detectors are set up to report only the first defect they find. Relative to actual defects, false alarms are rather common.
High car detectors and slide fences
Devices to detect cars or loads of excessive height or width, or shifted loads, are often placed in advance of bridges, tunnels, or other situations of reduced clearance. The growth in double-stack container and tri-level autorack operations has led to greater use of high-wide detectors.
Some safety detectors monitor the track, not passing trains. The most common of these, rock slide detectors (“slide fences”) are designed to break when hit by falling debris. When they do, an electrical circuit is broken, causing signals to go red on either side of the obstruction. High water indicators, earthquake sensors, and high wind detectors work similarly.
While the use of talking detectors has contributed to the demise of a cherished element of railroading – the caboose – they’ve been a boon to train-watchers with radio scanners. From detector reports, one can learn of approaching trains before they’re in sight, and acknowledgements can tell the listener what kind of train to expect.
Steam engines on excursions routinely set off detectors. Waivers are issued allowing them to proceed, providing the reported “defects” are near the cylinders or firebox.