Electronic or mechanical devices that detect problems with rail cars as trains pass them and provide a warning to the train or dispatch office. Common defect detectors are:
- Hot box detector – scans journals to detect overheated journals before they get hot enough to fail. The detector compares the relative temperature of the axles on either side and if one is significantly hotter than the other it trips. Modern systems can also track the temperature of axles of train across several detectors and if an axle begins to trend hotter, it will trigger a warning.
- Dragging equipment detector – detects objects or car parts being dragged under the train (typically brake rigging or derailed wheels.)
- Hi-wide detector – triggers an alarm if a car exceeds a certain width or height. Usually placed ahead of tunnels or bridges with tight clearance.
- Wheel impact detector – detects impact loads on the track, indicating flat spots or broken wheels (WILD – wheel impact load detector).
- Cracked wheel detectors – acoustically pings a wheel to detect cracked wheels.
Older systems (1960’s through 1980’s) alerted the crews through a trackside light display or an indication to the dispatch office. Modern systems (1980’s and newer) broadcast a radio transmission to the crew. Some reported to every train, some only broadcast if a defect was found (talk on defect only). When a train was alerted to the defect they normally would have to stop and inspect their train for the appropriate defect.