Train horn

A train horn is an air horn used as an audible warning device on diesel and electric-powered trains. Its primary purpose is to alert persons and animals to an oncoming train, especially when approaching a level crossing. They are often extremely loud, allowing them to be heard from great distances. They are also used for acknowledging signals given by railroad employees, such as during switching operations. For steam locomotives, the equivalent device is a train whistle. Since trains move on fixed rails, they are uniquely susceptible to collision. This is exacerbated by the train's enormous weight and inertia, which make it difficult to quickly stop when encountering an obstacle. Also, trains generally do not stop at level crossings, instead relying on pedestrians and vehicles to clear the tracks when they pass. Therefore, from their beginnings, locomotives have been equipped with loud horns or bells to warn vehicles and pedestrians that they are coming. Steam locomotives had steam whistles, operated from steam produced by their boilers. As diesel locomotives began to replace steam on most railroads during the mid-20th century, it was realized that the new locomotives were unable to utilize the steam whistles then in use. Early internal combustion locomotives were initially fitted with small truck horns or exhaust-powered whistles, but these were found to be unsuitable and hence the air horn design was scaled up and modified for railroad use. Early train horns often were tonally similar to the air horns still heard on road-going trucks today. It was found that this caused some confusion among people who were accustomed to steam locomotives and the sound of their whistles; when approaching a grade crossing, when some people heard an air horn they expected to see a truck, not a locomotive, and accidents happened. So, locomotive air horns were created that had a much higher, more musical note, tonally much more like a steam whistle.