Train horns are audible warning devices found on most diesel and electric locomotives, which includes passenger trains and other kinds of railway vehicles. The core purpose of using a train horn is to alert people and animals to the presence of a train, mainly when it is approaching a crossing. These horn devices are mostly used for acknowledging signals given by railway employees.
Train horns are also used for applying air brakes while standing; for releasing of air brakes and proceeding; for acknowledging a flagman’s stop signal; for acknowledging any signal not otherwise provided for; for backing up; inspecting train for a leak in brake pipe system or for brakes sticking; warning to people and animals; when running against the current of traffic; when approaching stations, curves, or points where view may be obscured; when approaching and crossing other passenger trains or freight trains.
Train horns are operated by compressed air which is usually between 110 to 150 PSI and is given supply from the locomotive’s main air reservoir tank unit. The air flow throughout the train horn produces an action which is known as oscillation. Oscillation in a train horn is accomplished via a diaphragm assembly enclosed within the power chamber. As soon as the air is applied to the train horn the diaphragm vibrates against a nozzle which produces a sound.
Train horns are mostly mounted either on top of the cab roof or in front of the cab near the bell. On locomotives with high short hoods the air horn can be located away from the operating cab. Whereas on newer locomotives the horn is mounted at top of the engine compartment due to noise complaints from the train crew.
Similarly the meanings of horn signals vary from country to country. Most countries with former of current state-of-the-art railway networks such as Great Britain, France and Germany use standardized meanings of horn signals on their networks. While in other countries like United States and Canada meanings of horn signals can vary between individual railroads. For example in United States the pattern of blowing the horn remains two long, one short and one long, which is repeated as necessary until the lead locomotive fully occupies the crossing. Most locomotive engineers retain the authority to vary this pattern as necessary for crossings in close proximity and are allowed to sound the horn in emergency situations no matter where the location.
Train horns now are often used for non-railroad use as well. Enthusiasts quite frequently attend organized events where specially modified motor vehicles perform runbys.
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