Nathan Manufacturing, Inc. is a division of Micro Precision Group which manufactures Airchime, Ltd. train horns mainly for North America. It is one of two major train horn manufacturers in the United States, with Leslie Controls, Inc. being the other.

Robert Swanson founded Airchime Ltd., beginning by making custom steam whistles in his British Columbia home. He preferred the sound of steam whistles over the single-chime horns made by Leslie and Westinghouse Air Brake Company. In 1949, he introduced the Hexatone H5, of which some odd (88) were made according to Robert Eugene Swanson's personal manufacturing/sales records. The H5 was only preceded by the H6, of which (4), including a cast iron H6 prototype known as the 'Iron Maiden', were only produced and never sold in any quantity to any railroad and were considered experimental. The H-series and N-series, also rare, preceded the M-series manufactured by Nathan in the United States and Airchime Mfg. Co. of Vancouver, B.C., Canada. Airchime Mfg. Co's M3H became the standard 3-chime horn used in Canada. The Holden Co. Ltd. was the Canadian Distributor of Airchime Mfg. Co's locomotive air horns and steam whistles and did not manufacture any horns or whistles, a very common misconception.

Captain Charles Benter, who followed John Philip Sousa as Marine Corps Band conductor, played a role in developing what is called "the most melodic horn in railroading". The five-chime horn Swanson and Bentor came up with became the H5 and H3-series horn, characterized by "short, fat bells that were welded together by their flares, the power chambers not being on an even plane, and copper air tubes supplied air to all five bells from an air-chest below the largest bell. The H3 had three chimes, and the H5 five chimes. This is the horn that Bentor had Swanson modify to A 7th major from C# diminished 7th.

The H5 and N3 were evolved into the superior M-series horns. (M standing for "Modulation") Both the 3-chime and the 5-chime had all power chambers on an even plane, and the M5 eliminated the air tubing, instead using a manifold that internally supplied the air to all 5 (or 3) bells. The M5 is still to this day considered by historians and collectors as the most musical of all locomotive air horns. Although supposed to have been originally tuned to A 7th major, 1st inversion with doubled third, few if any M5s ever blew that diatonic chord. M5 bells tended to be varying amounts of hertz off from the diatonic keyboard, just like a chime steam whistle, so about any sound close to various chords could be heard on a healthy M5.

In the early 1950s, Swanson introduced the Nathan Truck Horn in T-E and T-5 versions. In 1953, the truck horn was refined for use on locomotives, becoming the P-series. Swanson, whose Airchime never made the horn, sold the rights to Nathan. He never liked the P-series considering the K-series to be the ultimate horn, with the P (President's whistle) being a cheaper alternative. The P-series did its job, and was easier to repair than the M-series because each M-series bell had a separate diaphragm, while the P-series horns had the same diaphragm for each bell, a practice Leslie was already using. M-series horns also needed more frequent maintenance. The P-series was Nathan Mfg. Co's equivalent to the Leslie SuperTyfon.

All generations of P-series horns "used a steel diaphragm disk. Also, the orifice opened up into a fairly wide, oval-shaped hole in the horn's internal chamber." The second generation P-series diaphragms remained the same stainless steel, however the inlet to the power chamber was now round, due to elimination of the core in the casting pattern. The changes made no difference in the sound.

The P3 was used by the first diesel locomotives of Illinois Central and Southern Pacific. The P5 was used for the passenger trains of Illinois Central, Rock Island and Southern Pacific, along with various freight units, especially those on the old Southern Railway.

In 1954, the K5H/K3H made their debut, using a D-sharp minor chord (D-sharp, F-sharp, A-sharp) because of Canadian regulations. "H" stood for "high-pitched" because none of the low-pitched bells available were used. Later, "H" referred to high-profile manifold, while L stood for low-profile. The K5H is Swanson's best yet imitation of a steam train chime whistle, heard at a distance it was described as "unresolved" and "haunting". It was used by Norfolk Southern and CSX (some of whose older engines still use it), and by C&NW. Early K-series horns were sand-cast, like the P-series, but later ones would be die-cast.