We’ve all heard it, that penetrating steady tone blast from a passing locomotive. It is designed to warn motorists or anyone in the right-of-way to stand clear, that danger is imminent for the unaware motorist or passer-by.

Although air horns are used in emergency vehicles and trucks, it’s the train whistle that stirs the imagination. Like trains in general, these tones stir a strong nostalgic response in those who grew up near a railway. It was Hank Williams that so aptly described in song that "lonesome whistle blow." Many youngsters have grown to adulthood with their fondness for diesel train horns intact. Today, there are many hobbyists who avidly collect these horns and refurbish them. The internet has allowed these collectors to band together to promote and share their hobby. Common collectible train horn manufacturers include Nathan, Leslie, Wabco and Prime. Only Nathan Manufacturing, Inc. and Leslie Controls, Inc. are still in existence today.

All air horns operate in similar manner. Imagine for a moment a megaphone type loudspeaker. It has a magnet, a voice coil and a diaphragm. After an applied electrical input, the diaphragm vibrates causing sound waves. The air horn is a type of speaker too but is actuated by air rather than an electrical/magnetic current. Air pressure causes the diaphragm to rapidly open and close the airway many times a second. This is heard by the ear as a steady tone.

Many hobbyists can determine a train horn model simply by listening to its tone. The pitch of a train horn is affected by many variables. The diameter of the air inlet orifice determines the loudness. The internal components and the bell horn material also affect whether the tone will be mellow sharp or in-between. The size and shape of the bell horn also changes the tone.

Air horns are supplied by an air tank. The tank holds a finite quantity of air so it needs to be re-filled by an air compressor. True diesel air horns need plenty of compressed air so hobbyists recommend a minimum 20 gallon tank. Compressors can run the gamut from small 12 volt electric compressors to moderate size gasoline compressors to large 10HP gas behemoths.

Truck and SUV owners represent a growing market for air horns. These vehicle owners are drawn to the flashy chrome appearance of these horns. Some hide the horns so as not to attract attention. These scaled down versions are typically bought new in a kit for easy installation. A consumer can choose a truck air horn, typically in a single or two horn model, or a train horn that usually contain three or four horns that produces that distinctive ominous chord blend.

The kit typically involves a 12 volt compressor running off the battery and a one to three gallon steel air tank running to a high flow solenoid valve. Separate tubes then run to each horn in the system. These kits can also be installed in a car or boat.

A collector usually mounts an air horn on a truck or truck bed for demonstration purposes and to test different configurations. If you are considering purchasing a train horn kit for your personal vehicle, check with your local ordinance. Train horns easily produce sound levels of 170DB. Remember, these horns systems do not take the place of your regular electronic horn so there is no need to draw attention in ordinary situations. The loud train horn blast is best left for a true emergency or for use at an isolated location.