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.