Hot box

A hot box is the term used when an axle bearing overheats on a piece of railway rolling stock.

The journal bearing was replaceable, but if neglected, it would heat to a temperature where the babbitt bearing alloy would melt away, leaving the brass carrier riding on the steel axle, and result in a "taper journal".

This would eventually lead to the axle fracturing and the car above falling onto the wheel, or failure of the taper journal, causing the side frame and journal box to fall below the level of the rails, either of which could cause a major derailment of the train.

They would then sound the audible report "All Black" to mean the train was not giving off any light energy that would indicate combustion or destruction of the wheel bearings.

Most of the larger railroads use defect detectors to scan passing trains for hot box conditions.