In the railroading community in the United States, the term "hot rail" is used in two different ways. In both cases, the term is used to convey a sense of immediate danger so that people are aware of a situation which may be unsafe. A hot rail may refer to a track about to be in use, or an electrified rail. When people are warned away from a hot rail, they should comply, as they could be at risk of serious injury or death.
In the first sense, a hot rail is a length of track which is about to be in use by a train or maintenance vehicle. Especially in switching yards, people may refer to hot rails frequently as trains are moved around, usually using a track number to alert people as to which rail in the yard is about to go hot. This is designed to allow people to time to move away so that a train can pass. In rare cases where a train is actively using a section of track and someone shouts out a hot rail warning, it gives the train crew time to issue a warning in return to ask the other train to wait so that they can clear the track.
While trains are famous for being very noisy and they tend to cause agitation in the tracks as they approach which can alert people to an oncoming train, sometimes a train can sneak up on people. This is why hot rail warnings are important. Especially since many workers in train yards wear ear protection so that their hearing is not damaged by their loud working environment, it is critical that people near the tracks be alerted both verbally and visually when a train is on the approach so that they have time to avoid it.
The other type of hot rail is an electrified rail which is used to deliver power to trains. Many subway and metro systems power their trains with a hot rail in this fashion. Such rails are very dangerous to touch and are usually shielded to reduce the risk of injury. They are said to be "hot" in the sense of "live with electricity," a slang term which is also used in other settings related to electricity in the United States.
Usually signage indicates the presence of a third rail and warns people away. Such signs are often graphic in addition to textual so that people do not need to be able to read the sign to understand that there is a danger in the area.