A hot box is the term used when an axle bearing overheats inside a railroad car’s Journal Box. The Journal is a cast to the truck frame box in which the axle bearings were housed in. It used oil-soaked rags or cotton packing to reduce the friction of the axle against the truck frame. When the oil leaked or dried out, the bearings overheated often starting a fire or melting and locking up the wheel, causing a derailment or burning the train car if not detected early enough.
That…is a HOTBOX
"Look at that trail of smoke coming from the locomotive, looks like she has a hotbox"
"I told yah to oil the bearings...now we have a hotbox!
Guy 2: "SWEEET!"