A hot box is the term use when an axle bearing overheats on a piece of railway rolling stock. The term is derived from the journal-bearing trucks used before the mid 20th century. The axle bearings were housed in a box that used oil-soaked rags or cotton (collectively called 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 that could destroy the entire railroad car (and cars coupled to it) if not detected early enough.