Doug,
"Since roller bearings are used almost exclusively today, the hotbox is a dead term on a mainline railroad."
Electronic Hot Box detectors are very common sights on all the main lines in the USA. If a roller bearing fails the box will get hot.
Any time the UP sends out the 3985 or the 844 they have to give special instructions to the dispachers to ignore the hox box detectors for the first 20 or so axles on the train.
A friend of mine used to work for the Southern Pacific and he had a hot box on a loaded box car. He said the bearing was so he could see the white hot glow from the rollers through the side of the journal box. He said the box was slowly oozing into a blob.
Back in the day of friction bearings a hot box could get up in temperature enough to melt the brasses and then the brass would drip out onto the ties and set them on fire. It could also get hot enough to combust the grease in the box sending flames under the car, or if the lid was opened let out flames that would burn the side of the car.
Cheers,
Curtis F.