Hot Bearing Detectors used to be the human beings in the signal towers along the track. They had special hand signals to tell the crew in the caboose if the train had a problem.
Pinching your nose and pointing to the wheels meant something was hot (a bearing). At one time they had "stink bombs" in the journal box that would smell something awful when they got too hot.
Holding one hand palm up and moving your other hand palm down above it from back to front meant a brake was sticking and the wheels on a car where sliding. Sliding wheels could overheat and cause problems.
On the NYCRR there was a hot box on an eastbound freight in Syracuse NY about 1944, the axle end burnt off and the train derailed and destroyed the SS (Signal Station = Signal Tower) and sadly killed the tower operator (The Human Hot Box Detector).
North of Philadelphia (Frankfort Junction) a Northbound Pennsy Passenger train (loaded with people, it was during WWII, Labor Day 1943) had a hot box, the tower operator spotted it and called the next tower ahead to drop the signal and cause the train to stop. Too late, the axle failed and the train derailed. The heavy coaches overturned and took out a signal bridge and some of the catenary supports, large loss of life. Same spot as the Amtrak train went through the 40 mph curve at 90 mph and derailed and killed some folks in 2015.
https://en.wikipedia.org/wiki/1943_Fran ... rain_wreck