Friction bearings

What constitutes a "hotbox"?
Once upon a time, I remember train crew members and car knockers testing for hotboxes with their hands. Almost always with gloves.
At what level of pain to the human hand did a bearing get inspection of the axle while the old brass was out, then a new brass, plus waste and oil.
Nowadays, our passenger cars and remodeled freight cars are way below loaded freight car weights. Do these cars, with reasonable maintenance, ever have bearing failure?
Related question: Are friction bearings inspected with mechanic's temperature sensing "guns"?