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{{Other uses|Hotbox (disambiguation)}}
[[File:Railroad truck, FM55-20.Fig8-8.png|thumb|right|250px|US-style [[Bogie|railroad truck]] with [[journal bearing]]s inside [[journal box]]es]]
A '''hot box''' is the term used when an axle [[Journal bearing|bearing]] overheats on a piece of [[railway]] [[rolling stock]].<ref>{{cite patent|US|4659043}}</ref> The term is derived from the [[Journal bearing|journal-bearing]] [[bogie|trucks]] used before the mid-20th century. The axle bearings were housed in a [[Journal box|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.

The packing and bearing had to be regularly inspected by yard crews, and packing was often added at major stops. The [[Plain bearing#Journal bearings|journal]] bearing was replaceable, but if neglected, it would heat to a temperature where the [[Babbitt (metal)|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. Train worker duties consisted partly of inspecting the train as it ran by, looking for smoke, sparks, or fire. 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. If the train worker saw "Red" or smoke, he would alert other crew members, or else make an emergency stop to the train to prevent further damage.

When this type of [[axle box]] was used, any [[diesel exhaust]] smells had to be tracked to their source, as a hot-box sometimes smells similar. Most of the larger railroads use [[defect detector]]s to scan passing trains for hot box conditions. Some of these detectors also have "automated mile posts" which send an automated radio signal to the train crew listing the train number, track number, number of axles on the train and train speed.

Modern [[Ball bearing|ball]], [[Rolling-element bearing#Roller bearings|roller]] or [[Tapered roller bearing|tapered bearings]] can also overheat, but the likelihood of a roller bearing overheating is usually far smaller than with journal bearings. When modern bearings do fail, the balls or rollers and their [[Race (bearing)|races]] fail, generating heat which can ignite fires or be the ignition source of a [[dust explosion]] in grain, coal, sawdust, etc.

== Gallery ==
<gallery class=center>
File:Phoenix 2FBOA Kopie.png|Hot box and hot wheel detection system PHOENIX MB (voestalpine SIGNALING Siershahn GmbH)
File:2020 at Cogload Junction - hot axle box detector.JPG|Hot axle box detector at [[Cogload Junction]], England
File:GWR Spoked wagon wheels.jpg|A [[Wheelset (rail transport)|wheelset]] from a [[Great Western Railway|GWR]] wagon showing a plain, or journal, bearing end
File:Journal box.png|[[Journal box]]
File:Achslager-Nordbahn-Wagen.JPG|A [[Swiss Northern Railway|Swiss]] journal box
</gallery>

== See also ==
* [[Hightstown rail accident|1833 Hightstown rail accident (United States)]]
* [[Frankford Junction train wreck|1943 Frankford Junction train wreck (United States)]]
* [[1979 Mississauga train derailment|1979 Mississauga train derailment (Canada)]]
* [[Summit tunnel fire|1984 Summit tunnel fire (England)]]

== References ==
{{reflist}}

== Bibliography ==
* Railway Age magazine April 1, 1957
* Model Railroader 1980 May 62pp

[[Category:Rolling stock]]

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