Train horn

A train horn is an air horn which serves as an audible warning device on diesel and electric locomotives. Its primary purpose is to alert persons and animals to the presence of a train so they can clear the tracks, especially when approaching a grade crossing. They are also used for acknowledging signals given by railroad employees (i.e. during switching operations).

History and background

Railroad trains have a much greater mass and thus a longer stopping distance than road vehicles. They typically cannot stop in time to avoid hitting obstructions on their tracks. Trains do not stop at grade crossings, where roads cross tracks, but rely 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 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 efficiently 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 so the air horn design was scaled up and modified for railroad use.