Diesel Locomotive Airhorn History

by Brent Lee, Official SoundTraxx Hornologist

Throughout the history of locomotive airhorns there have been four major players in the airhorn business: Nathan, Leslie, Westinghouse and Prime. From the early days of Westinghouse single notes on locomotives to today’s Nathan K5LA Airchime five chimes, there have been several horns that have come and gone. Due to changes in production, i.e. different kinds of metal used in production, or changes is the castings being used, a locomotive airhorn can change its tune, pitch or sound drastically from one to another.

Airchime (Nathan and Holden)


Both Nathan and Holden produced the Airchime M model. Robert Swanson, of Airchime, introduced the M horn in 1949. While the H and N series horns were the foundation of the future standard M horn, very few were made and there are even fewer in existence today. The M horn was the first production horn made by Robert Swanson.

M3 (early): This is the early M3 horn that was produced for a few years up to the early or mid fifties. It has a round base around the bells, and 3/16 air orifices that produce a very melodic and rich three-chime sound. The later version has a scallop base around the bells. This was done to cut back on the amount of aluminum used in the horn. This horn has the larger and louder 5/16 air orifices in the diaphragm head. It can be recognized by its being very loud, due to the increased volume of air that the diaphragms get, as compared to the 3/16 horn. The M3 horn has #1, #2 and #4 bells.

M3H: Made in Canada by Holden Ltd., this horn is similar to the M3. The big difference between the two horns is the bells. Instead of the 1, 2 and 4 combination, the M3H has #2, #3 and #4 bells. The same power chambers were used as the standard M3, with the #2 bell having a #1 power chamber, the #3 having a #1 power chamber and the #4 bell retaining its #4 power chamber. This horn was the everyday common three chime in Canada.

M5: The most famous five-chime horn ever, this horn was used on several class one railroads on their passenger and freight locomotive fleet. Introduced in 1949, Nathan only produces this horn by special order, and more often than not, the castings are done only once a year. The later version M5s were similar to the M3 horns, with a scallop base to reduce excess aluminum in the casting to cut costs in production. 


The Airchime P model horn was introduced in the early 1950s. Manufactured by Nathan, it was designed to be an easier horn to work on compared to the hand tuned M horn. The P horn had a common diaphragm that could be purchased for any of the bells in case one should become defective. (The M horn had a different sized diaphragm for each bell, and Nathan’s competitor, Leslie was already producing a horn with common sized diaphragms.) The P horn looks drastically different from the M horn. M bells are short and fat, where a P bell is long and skinny. P horns are known by horn enthusiasts to have two to three generations. This is because of the difference in sound over the years produced. The Nathan P5 Airchime was used by the Illinois Central, Rock Island and Southern Pacific on passenger units and is truly one of the most loved horns of all times. In about 1977, the castings were changed on P horns and the horn took on a whole new sound. This can be identified by a casting mark on the left side of the bell, right above the bolthole. Older P horns are marked with “PAT. PENDING” and newer castings have “PAT. PEND.” There is a big difference in the sound of the two castings, with the older ones having a rich, melodic, organ sound, and the new castings having a slightly distorted sound compared to the older castings.

P3: The early fifties P3 like those used on early Illinois Central or Southern Pacific first generation diesels has #1, #2 and #4 bells.The modern version of a P3 is not as rich as the early horns.

P5: The early fifties P5 had an incredibly melodic tone The P5 has #1, #2, #3, #4 and #5 bells. The modern version P5 has the so-called “chirp” right before it blows. A unique 1953 version P horn seems to be a bit of an enigma. It has #0, #1, #3, #4 and #5 bells instead of the common #1, #2, #3, #4 and #5 bells. It has a very distinctive sound.


The K model was first introduced in Canada around 1954. The first models had adjustable back caps for tension on the diaphragms for proper sounding from low to high pressure. The early K models were also sand cast like the P model horn. Later, these horns would be die cast. Today, the K model horn is the most used horn in North America. The Nathan K horn has taken the place of the Leslie S5T on the CSX, with the K5LA the standard on all CSX locomotives. The one K horn user that will be remembered in history will have to be Amtrak, the K5LA being the voice of the Amtrak F40PH. The K5LA was created for Amtrak, and the F40s were the pride of Amtrak and Amtrak fans. With their sleek look and new sound, fresh to railroaders and railfans everywhere, this beautiful tone could only be Amtrak - unless you were in Chessie System territory. The horn was created for Amtrak, but Chessie System actually received the first production K5LA. The K5LA is the only horn to ever have a steam whistle made to sound like it. When Chessie System had their Chessie Safety Express, they wanted a whistle to sound like the new K5LA used on their diesels. So Robert Swanson of Airchime, who had re-tuned his K5H to create the K5LA, made a whistle for Chessie’s steam locomotive and one for his locomotive, the Royal Hudson. These two steam whistles sounded identical to the K5LA airhorn.

K3L: Holden Ltd. of Canada produced this horn. It is a sand cast horn with #1, #2 and #3 bells. CP Rail and Canadian National used this horn. Today’s version is a die cast horn manufactured by Nathan. The L designation means low profile base.

K5H: This horn was first used in Canada and later used by American railroads in the 1980s. The NS, CSX and C&NW all used this horn. It can still be found on NS and CSX units. This model horn has the five basic K bells designed in the 1950s, using the K model #1, #2, #3, #4 and #5 bells.

K5LA: The very early 1970s model from Amtrak came equipped with snow cones. This is a Amtrak add on to prevent debris entering the throat of the bells. Nathan was producing two different #5 bells at one time, one a standard and one for a railway in Africa. It was decided to eliminate one bell to save costs and keep the South African Railway F bell. They now hollow out a chamber in the F bell to make it a D#, which gives  the proper chord. A second version had a #3A bell marked 3A, but the #4A bell is marked 4, which is a early bell before they began marking them. The sound is a standard K5LA chord. The modern everyday K5LA has marked #3A and #4A bells, and the #5 is a hollowed out F to D# conversion bell. This is the horn on a new CSX, NS or Amtrak locomotive.


Leslie has been in the horn business for years. Leslie produced the most famous of all single note horns. It was the A200 model. Used on locomotives from GG1s to GP7s, it was the single note horn remembered by all, often referred to as the “BLAT” horn because of its sound. The A series had different sized horns, ranging from large to small. Leslie tried to market a chime horn using A model bells of different lengths on a common base, but there were very few sold compared to other manufacturers.

In 1951, Leslie introduced their Leslie Supertyfon S series five-chime horn. It had standard diaphragms for all bells. This was different from the Nathan M5 because the M5 had a different sized diaphragm for each bell. The S series made it easy to work on and use in two respects; it had the same sized diaphragms and did not have to be tuned or adjusted like the M horns. The Leslie S series horn was an instant success. Many roads soon became users of the Leslie S3L and S5T. During the mid 1960s, Leslie made a change in their castings on three bells. Leslie had been making their #25, #28, and #31 bells in two pieces. A decision was made to eliminate the two-piece bells for easier construction. Leslie decided not to make the #28 bell anymore and recast the #25 and #31 bells. But there was one problem - the #31 bell came out fine, in tune, with the same sound as the original two-piece #31. The problem existed in the #25 bell. It came out as a discord and out of tune. Over thirty years later, Leslie still manufactures the #25 bell that is a discord. It has become one of the favorite horns of railfans and horn collectors, even with its discord. Both the S3L and the S5T use this bell.

Leslie went on to make their next power chamber, the RS. The RS power chamber has fewer parts than the S model head. The old S model heads had two-piece diaphragms. Both made of stainless steel, they had the diaphragm and then a diaphragm ring that went on top of the diaphragm itself inside the power chamber. The RS model has a single stainless steel diaphragm with a rubber gasket around the outside of it. Usage of the S model power chamber is becoming a memory with the RS models now replacing the older S heads when horns are overhauled.

A200: The A200 is the most famous single note horn in the history of locomotive airhorns. Used on early electric and diesel-electric power, it is often referred to as the GG1 horn, and has been used by most class one railroads. It was often used with another A200 pointing in the opposite direction for back up movements. It has a rich tone, and a very well known sound. That is where it gets its nickname, the “BLAT” horn. Leslie made the A200 in both bronze and aluminum. The A200 was also used on steam locomotives. The Milwaukee Road and the Southern Pacific both used them. The SP 4449 is a good example of a steam locomotive with a bronze A200.

A125: The A125 is smaller than the A200. It was used on switch engines and some early road power. It was often used along with the A200 to create a two-chime horn. It can be seen on some EMD E and F units most of the time reversed on the Fireman’s side. The A200 would be on the engineer’s side facing forward. The A125 came in both bronze and aluminum. The A series had to be tuned and would produce some very strange sounds if the horn was a little out of tune. One of the sounds the out-of-tune A125 would make was called the “Billy Goat”. At full volume the horn would make a sound just like a goat. This was caused by the tension being slightly off on the diaphragm head. The A125 could produce a very warm, nice chord when properly tuned, and had a higher sound than the A200 because of its smaller size. EMD always seemed to favor the Leslie A series for single note horns compared to Westinghouse’s E2 or A2 models which are similar to the Leslie A200 and A125 respectively.

S2M: The S2M horn from Leslie was a two-chime horn with #31 and #44 bells.

S3L: The S3L is probably one of the most widely used locomotive horns in the world.  The S3L has #25, #31, and #44 bells. It has a very pleasant chord. The S3L is most commonly used with the #44 reversed. The B&O reversed the #31 bell, and a few roads like PRR and SAL left all the bells forward on some units. The mid 1960s version has the one-piece bells that produce a discord in the #25 bell. It is probably the most recognized S3L chord there is.

RS3L: The RS3L is the most modern S3L currently produced by Leslie. It has the new RS power chambers. The RS heads can be recognized by the spike on the back of the heads. The RS uses a new designed diaphragm that is one piece, compared to the older two-piece assembly of the S head. The RS has the single diaphragm with a rubber gasket around the outside of it. The RS3L has the one piece bells with the #25 bell being cast out of tune.

S5T: The Leslie Supertyfon has a long history. With Leslie’s introduction of their five chime Supertyfons around 1951, the S5T became one of their best selling horns. With railroads like GM&O and the Frisco using the S5T as a standard application, the S5T quickly became a horn everybody loved. GM&O had used everything from Leslie A200s, A125s, Nathan M5s, P5s, and even Westinghouse A2s. When their second order of GP30s came out they decided on Leslie’s S5T chord and went with a split five chime utilizing Leslie’s two and three chime horn combination to achieve this. It made for a beautiful sound, and an awesome appearance. When the GM&O GP35s came onto the property, they were equipped with Leslie S5Ts, and even stranger, they had the #25 bell reversed. No other railroad ever did this except for the GM&O. The S5Ts on the GP35s were the original two-piece bell horns that had a sweet sound. SD40s and GP38s purchased later would also have the Leslie S5T. All of these horns would contain the one-piece bells with the discord. Most of them would also have the odd reversal of the #25 bell, and a few had the standard factory reversal of the #31 and #55 bells.  Still today on the Illinois Central, a few GM&O Leslies can still be found. Frisco’s had A200s, M5s, E2s, and even one Hancock model 4700 air whistle. But the Frisco decided on the Leslie S5T early on, and it was their standard horn up to the very end on merger day, November 21, 1980. They also went through the transition of two-piece bells to the later one-piece bells. The S5T has #25, #31, #37, #44, and #55 bells. It is truly a beautiful horn.

RS5T: The RS5T has one-piece bells and RS power chambers. The RS heads produce a slightly different sound than the round head S5T. The RS heads tend to chirp when the whistle valve is opened.

Westinghouse (Wabco)

Mention Westinghouse to most railroad people and they think of air brakes, but Westinghouse has also made locomotive airhorns for years. Westinghouse never had as many different examples as its competitor companies, but the few they produced were widely used. The most famous Westinghouse horn is the single note E2. It is the largest bell produced by Westinghouse.

When chime horns became big, Westinghouse decided to take their E model horn and create a chime version. The result was three different melodic horns, the E2B1, E2B2, and the E2B3. The most common of the three used by railroads was the E2B1. Even the numbers of E2B1s sold are very slim, compared to other three chimes of different manufacturers, like the Nathan M3. Pennsylvania had some Westinghouse E2B1s, but it is still hard to find pictures of PRR units with them.

A2: Another interesting horn is the A2. The A2 is Westinghouse’s small switcher horn. The A2 was used mostly on switch engines or small industrial locomotives, usually a small ALCO or GE product. There were variations of the A model horn also. The most famous is the AA2, a two-chime model, is still manufactured by Westinghouse and is known for being used on interurban equipment.

E2: The Wabco E2 is a very interesting horn, and when properly tuned has a unforgettable sound. It is often mistaken for Leslie’s A200, or vise versa. The Wabco E2 has a longer, but thinner bell than the Leslie A200, and the back cap looks to have little teeth all the way around it. Those are used to lock down the back cap when tuning. The A200 has the tension bolt on top, and the four big teeth sticking out of its back cap. Once you recognize these differences between the two, they are more than just simply “BLAT” horns. The E2 has a very complex diaphragm assembly and the diaphragms are famous for cracking. Many railroads used the Wabco E2. It was often seen on ALCO locomotives. ALCO seemed to prefer the E2 as a standard application single note horn on road units. Like the Leslie A200, the E2 was often used in pairs, with one horn reversed.