During the last century when diesel locomotives began to replace steam on most railroads, it was realized that the new locomotives were unable to efficiently utilize the steam whistles then in use. The early internal combustion locomotives at that time were 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, hence came the invent of train horns.
Train horns at one time were actually made for trains. The primary purpose of using these train horns was to make people and animals aware that a train is approaching. These horns were also used in trains for acknowledging signals with the railroad staff. Apart from these uses, the train horns were used for various signals which varied country to country.
Later the train horns were used in fire trucks, ambulances and other large vehicles that needed to warn others of their presence on the road, the reason being that they produced loud sounds to immediately clear their way on the roads. Perhaps this is the reason that earlier they were also used in emergency vehicles like ambulances and fire trucks. Most of the fire trucks in the United States still use the air horns with shutter tone.
There have been several manufacturers of train horns in North America over the years. Air Chime Limited was the pioneer in introducing the train horns by the work of Robert Swanson in 1949. Prior to the early 1950s, locomotives were equipped with air horns that sounded but a single note. Swanson, using ancient Chinese music theory, sought to develop an air horn which would mimic the sound of a classic steam whistle and managed to produce a six-note model which was known as the famous H6. Later Swanson refined his H6 model in H5, which gave the sound of a five-note chord.
In 1950, Air Chime came up with the M series, which was a further improvement on the earlier horns through elimination of unnecessary moving parts. Later on with the guidance of Swanson, Air Chime focused on ease of mass production, low maintenance and reliable air horn designs with the introduction of the P series in 1953 and K series in 1954.
After mid 1950s many other American companies including Gustin Bacon Manufacturing Company, Leslie Company, Buell Air Horns, Prime Manufacturing Incorporation and Westinghouse Air Brake Company introduced their range of train horns, which became quite popular in that era. The more popular of these were the ones made by Air Chime Limited and the Leslie Company.