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A train whistle or air whistle (originally referred to as a train trumpet or air trumpet) is an audible signaling device on a steam or gas locomotive, used to warn that the train is approaching, and to communicate with rail workers. Modern diesel and electric locomotives primarily use a powerful air horn instead of a whistle as an audible warning device. However, the word whistle continues to be used by railroaders in referring to such signaling practices as "whistling off" (sounding the horn when a train gets underway).
The need for a whistle on a locomotive exists because trains move on fixed rails and thus are uniquely susceptible to collision. This susceptibility is exacerbated by a train's enormous weight and inertia, which make it difficult to quickly stop when encountering an obstacle. Hence a means of warning others of the approach of a train from a distance is necessary. As train whistles are inexpensive compared to other warning devices, the use of loud and distinct whistles became the preferred solution for railway operators.
Steam whistles were almost always actuated with a pull cord (or sometimes a lever) that permitted proportional (tracker) action, so that some form of "expression" could be put into the sound. Many locomotive operators would have their own style of blowing the whistle, and it was often apparent who was operating the locomotive by the sound. Modern locomotives often make use of a push button switch to operate the air horn, eliminating any possibility of altering the horn's volume or pitch.