A hot box refers to the time of friction bearings and brasses. If not kept well lubricated, the rotation of the axle will heat up the brass in the journal box and get quite hot.
Since roller bearings are used almost exclusively today, the hotbox is a dead term on a mainline railroad.
Railroads that are historic and use older equipment (tourist) may still have hotboxes. The C&TS and the SV and the EBT among many others used brass in the journals and so do need to watch out for smoke coming from journals.
I have seen one or two on the mainline over the years. It is interesting to see smoke coming from a journal in the middle of the tran. This is one of many reasons a caboose was used on a train - to watch out for hot boxes.
Doug