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THE LOUDEST TRAIN HORN IN THE WORLD Train horns are operated by compressed air, typically 125-140 psi (8.6-9.6 bar), and fed from a locomotive main air reservoir. The flow of air throughout the horn produces an action known as oscillation. Oscillation in a train horn is accomplished via a diaphragm assembly enclosed within the power chamber. When air is applied to the horn, the diaphragm vibrates against a nozzle. The oscillation of the diaphragm against the nozzle produces sound.
The configuration and overall dimensions of the bell ("bell" being the correct term for the trumpet assembly) determine the fundamental and frequency produced (measured in hertz).
North American diesel locomotives manufactured prior to the 1990s utilized an air valve actuated by the engineer through the manipulation of a lever or pull cord. Use of this method made possible a practice known as "feathering", meaning that modulation of the horn's volume was possible through finer regulation of the air valve.
Many locomotives manufactured during the 1990s made use of pushbutton controls. In addition, several North American locomotives incorporated a sequencer pedal built into the cab floor beneath the operators position; that when depressed, sounded the grade crossing sequence.
Locomotives of European origin have featured pushbutton control of air horns since the mid-1960s.
Current production locomotives from GE Transportation Systems and Electro-Motive Diesel have utilized a lever-actuated solenoid valve. http://en.wikipedia.org/wiki/Train_horn