Train horn  A train horn is a powerful air horn that serves as an audible warning device on electric and diesel locomotives. The horn's primary purpose is to alert persons and animals to an oncoming train, especially when approaching a grade crossing. The horn is also used for acknowledging signals given by railroad employees, such as during switching operations. Trains move on fixed rails, making them uniquely susceptible to collision. This susceptibility is exacerbated by the enormous weight and inertia of a train, which make it difficult to quickly stop when encountering an obstacle. Also, trains generally do not stop at grade crossings (level crossings), instead relying upon 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 or 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. This is why most train horns have a unique sound, different from that of road going trucks, although many switch engines, which didn't see road service (service on the main lines), retained the deeper truck-like horns. Strict regulations specific to each country specify how loud horns must be, and how far in advance of grade crossings and other locations locomotive engineers are required to sound their horns to give adequate time to clear the tracks. Standard signals consisting of different sequences of horn blasts must be given in different circumstances. Due to the encroachment of development, some suburban dwellers have opposed railroad use of the air horn as a trackside warning device. Residents in some communities have attempted to establish "quiet zones", in which train crews are instructed not to sound their horns, except in case of emergency.