A “hotbox” is not a box at all.

The term hot box refers to a hot wheel journal which indicates impending failure of that axle.  In the old days of cotton packing and grease, such a journal would usually catch on fire.  Modern roller bearings simply shatter spectacularly.

Now-a-days, with no cabooses, the railroads rely upon track side detectors.  If the detector senses a hot journal, the train crew slows down and sets the car on a siding as soon as possible.  The dispatcher sends out a car knocker to jack-up the car and replace the axle.  The new axle, of course,  has new journals. Once repaired, the next available train picks up the car and takes it on it’s way.