Hot Box
A bearing on a railcar axle that has overheated, historically a leading cause of derailments and now monitored by trackside detectors.
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A hot box is an axle bearing that has heated to an abnormal temperature, almost always because lubrication has failed or a defect has developed inside the bearing housing. Before roller bearings became standard, friction (or "plain") bearings ran on a brass shoe lubricated by an oil-soaked waste pad, and a deteriorating pad would overheat the journal until smoke or fire became visible — hence the name. An untreated hot box could burn off the journal and drop the wheel set onto the track, causing a derailment.
Today, every Class I railroad and most main lines worldwide are protected by hot box detectors (HBDs) — trackside infrared scanners mounted between the rails or beside them, which read the temperature of every passing journal and announce defects over the road radio: "BNSF detector, milepost 412.6, no defects, total axles 312." When a defect is reported the crew is required to stop and walk the train to inspect the affected axle, and depending on temperature thresholds may have to set out the car before continuing. Modern systems also detect dragging equipment, high-and-wide loads, and acoustic bearing defects in addition to heat.
For trainspotters, the radio announcement from a detector is one of the most useful pieces of free information available at trackside: it confirms a train just passed, gives a milepost, and announces total axle count — enough to deduce train length and identify whether the consist matches an expected schedule.