A train horn is a powerful air horn that serves as an audible warning device on electric and Diesel locomotives. The horn's primary purpose is to alert persons and animals to an oncoming train, especially when approaching a grade crossing. The horn is also used for acknowledging signals given by railroad employees, such as during switching operations.
- 1 History and background
- 2 Operation
- 3 Placement on locomotives
- 4 Audio samples
- 5 Manufacturers
- 6 See also
- 7 References
- 8 External links
History and background
Trains move on fixed rails, making them uniquely susceptible to collision. This susceptibility is exacerbated by the enormous weight and inertia of a train, which make it difficult to quickly stop when encountering an obstacle. Also, trains generally do not stop at grade crossings (level crossings), instead relying upon pedestrians and vehicles to clear the tracks when they pass. Therefore, from their beginnings locomotives have been equipped with loud horns or bells to warn vehicles or pedestrians that they are coming. Steam locomotives had steam whistles, operated from steam produced by their boilers.
As Diesel locomotives began to replace steam on most railroads during the mid-20th century, it was realized that the new locomotives were unable to utilize the steam whistles then in use. Early internal combustion locomotives were initially fitted with small truck horns or exhaust-powered whistles, but these were found to be unsuitable and hence the air horn design was scaled up and modified for railroad use. Early train horns often were tonally similar to the air horns still heard on road-going trucks today. It was found that this caused some confusion among people who were accustomed to steam locomotives and the sound of their whistles; when approaching a grade crossing, when some people heard an air horn they expected to see a truck, not a locomotive, and accidents happened. So, locomotive air horns were created that had a much higher, more musical note, tonally much more like a steam whistle. This is why most train horns have a unique sound, different from that of road going trucks, although many switch engines, which didn't see road service (service on the main lines), retained the deeper truck-like horns.
Strict regulations specific to each country specify how loud horns must be, and how far in advance of grade crossings and other locations locomotive engineers are required to sound their horns to give adequate time to clear the tracks. Standard signals consisting of different sequences of horn blasts must be given in different circumstances.
Due to the encroachment of development, some suburban dwellers have opposed railroad use of the air horn as a trackside warning device. Residents in some communities have attempted to establish quiet zones, in which train crews are instructed not to sound their horns, except in case of emergency.
Recent years have seen an increase of horn theft from railroad property.
Train horns are operated by compressed air, typically 125-140 psi (8.6-9.6 bar), and fed from a locomotive main air reservoir. When the engineer opens the horn valve, air flows through a supply line into the power chamber at the base of the horn (diagram, right). It passes through a narrow opening between a nozzle and a circular diaphragm in the power chamber, then out through the flaring horn bell. The flow of air past the diaphragm causes it to vibrate or oscillate against the nozzle, producing sound.
Keep in mind that when an air horn is not operating and has no fluid pressure flowing through it, the interior of the power chamber housing is completely air tight, as the diaphragm disc creates a full air tight seal against the nozzle surface. Referring to the cut-away blueprint diagram of a conventional air horn power chamber on the right, when a constant stream of pressurized fluid enters through the small inlet at the bottom, the pressure in the power chamber increases as it is air tight internally. The pressure continues rising in Chamber 'A' until the air pressure overcomes the spring tension of the Diaphragm. Once this occurs, the Diaphragm is deflected back, and in such, is no longer sealed against the nozzle. From this, the interior of the power chamber is now no longer air tight, as the Diaphragm has deflected off the nozzle. As a result, the pressurized fluid now escapes out of the horn bell. Because the pressurized Fluid exits through the horn bell at a much faster rate than the fluid enters into the power chamber through the base air inlet, the air pressure in the power chamber drops rapidly. As such, the Diaphragm re-seats itself against the nozzle surface. This entire process is one cycle of the Diaphragm operating. In reality, this operation process occurs much faster in accordance to the frequency produced by the horn. The constant back and forth Oscillation of the Diaphragm creates sound waves, which are then amplified by the large flared horn bell. The length, thickness and diameter of the horn bell contribute to the frequency of the note produced by the horn.
When vibrated by the diaphragm, the column of air in the bell oscillates with standing waves. The length of the bell determines the wavelength of the sound waves, and thus the fundamental frequency (pitch) of the note produced by the horn (measured in Hertz). The longer the horn bell, the lower the note.
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.
On April 27, 2005, the Federal Railroad Administration (FRA), which enforces rail safety regulations, published the final rule on the use of locomotive horns at highway-rail grade crossings. Effective June 24, 2005, the rule requires that locomotive horns be sounded at all public grade crossings at least 15 seconds, but not more than 20 seconds before entering a crossing. This rule applies when the train speed is below 45 mph (70 km/h). At 45 mph or above, trains are still required to sound their horn at the designated location (usually denoted with a whistle post).
The pattern for blowing the horn remains two long, one short, and one long. This is to be repeated or prolonged as necessary until the lead locomotive fully occupies the crossing. Locomotive engineers retain the authority to vary this pattern as necessary for crossings in close proximity, and are allowed to sound the horn in emergency situations no matter where the location.
A ban on sounding locomotive horns in Florida was ordered removed by the FRA after it was shown that the accident rate doubled during the ban. The new ruling preempts any state or local laws regarding the use of the train horn at public crossings. This also provides public authorities the option to maintain and/or establish quiet zones provided certain supplemental or alternative safety measures are in place, and the crossing accident rate meets government standards.
Common horn signals
The following are the required horn signals listed in the operating rules of most North American railroads, along with their meanings. Signals are illustrated by an "o" for short sounds, and "–" for longer sounds. (Note that these signals and their indications are updated to reflect modern practice.) Those rules marked with an asterisk (*) must be sounded when and/or where applicable. Those signals without an asterisk convey information to employees; they must be used when voice communication is not available.
|14 (a)||o||Applying air brakes while standing.|
|14 (b)||*||– –||Proceeding. Releasing air brakes. This signal is often referred to as "whistling off," despite it being given by an air horn.|
|14 (f)||*||o o –||Acknowledging a flagman's stop signal.|
|14 (g)||*||o o||Acknowledging any signal not otherwise provided for.|
|14 (h)||*||o o o||Backing up.|
|14 (j)||o o o o||Calling for signals.|
|14 (l)||*||– – o –||1. Trains or engines approaching public highway grade crossings shall sound the horn at least 15 seconds, but no more than 20 seconds before the lead engine enters the crossing. Trains or engines travelling at speeds greater than 45 mph shall begin sounding the horn at or about, but not more than, one-quarter mile (1,320 feet) in advance of the nearest public crossing. Even if the advance warning provided by the horn will be less than 15 seconds in duration. This signal is to be prolonged or repeated until the engine or train occupies the crossing; or, where multiple crossing are involved, until the last crossing is occupied. 2. Approaching tunnels, yards, or other points where railroad workers may be at work. 3. Passing standing trains.|
|14 (m)||*||o||Approaching passenger station.|
|14 (o)||o –||Inspect train for a leak in brake pipe system or for brakes sticking.|
|14 (p)||*||Succession of sounds||Warning to people and/or animals.|
|14 (q)||*||– o||When running against the current of traffic: 1. Approaching stations, curves, or other points where view may be obscured: and 2. Approaching passenger or freight trains and when passing freight trains.|
Exception: Engine horn signals required by rules 14 (b) and 14 (h) do not apply after momentary stops in continuous switching movements.
According to section 11 of Transport Canada's Locomotive Design Requirements, all Canadian-owned passenger train locomotives must be equipped with a dual-tone horn capable of producing a soft sound in normal operating mode and a loud sound in emergency situations. To comply with federal requirements, passenger railways use the Nathan K5CA-LS. This horn has two different air chambers, allowing the engineer to choose between sounding three chimes in "soft" mode or all five chimes in "loud" mode. The "loud" mode is intended for emergency situations, such as when a person or vehicle is on the tracks in front of an incoming train. The loud emergency mode produces a high-pitched and extremely discordant sound to get people's attention.
To maximize sound output, Transport Canada requires that all train horns be mounted facing the direction of travel, near the front of the roof, no further than 1.5 meters behind the rear of the cab, and near the centerline of the locomotive in a location where it will not obstruct exhaust pipes in any direction.
Horn warning signals
According to the May 2018 version of the Canadian Rail Operating Rules (CROR), specific train horn warning signals must be sounded as per rule 14. Like the American railroad authority, signals are illustrated using "o" for short sounds, and "_______" for longer sounds. In the CROR, it states that warning signals "should be distinct, with intensity and duration proportionate to the distance the signal is to be conveyed". The following table lists the train horn warning signals required by Transport Canada. The signals marked with an asterisk (*) must not be replaced with radio communication.
|14 a)||o||When standing, to indicate that braking system is equalized, angle cock may be closed|
|14 (b)||o o||To answer a "stop" signal (except for a fixed signal)
To answer any signal not otherwise provided for
|14 e)||o o o o o o||To notify crews of fire on the tracks, to be repeated as often as required|
|14 f)||*||succession of short sounds||To warn people to get off the tracks|
|14 l)||*||____ ____ o ____||To warn people at a level crossing that a train is coming. An indicator, located a quarter mile from the crossing, marks the location where the horn must be sounded when the train is travelling faster than 44 mph (70 km/h). When the train is travelling at 44 mph or less, this signal must be sounded for twenty seconds prior to occupying the crossing. The signal must be prolonged or repeated until the crossing is fully occupied by the lead unit of a train.|
|*||To be sounded at specific indicators located along the tracks in special instructions|
|*||To be sounded at frequent intervals when view is restricted by weather, curvature, or other conditions|
|Special instructions are given when this signal is not required to be sounded in whole or in part|
|14 t)||When snow removal equipment is being operated ahead of a locomotive, the snowplow operator is required to sound rules 14 f) and 14 l). The engineer operating the locomotive shoving the snow removal equipment is required to sound all other horn warning signals as per rule 14.|
Noise from train horns
Residents living in close proximity to train tracks may be disturbed by the sounding of train horn warning signals. However, railroad engineers are obligated to sound their horns at all times, which may lead to noise complaints. Transport Canada allows municipalities to pass bylaws that prohibit train horn sounding at train stations and level crossings, as long as Transport Canada grants approval to that municipality .
Horn signals are regulated in the Zp category of the Eisenbahn-Bau- und Betriebsordnung. Their most common use today is when approaching a level crossing that lacks barriers, and for warning purposes.
Whistle posts are labeled with the letter "P" (for 'Pfeifen'). Common signals are:
|_||Zp 1 - Generic "Attention" signal.|
|o||Zp 2 - Tighten handbrakes.|
|o o||Zp 3 - Strongly tighten handbrakes.|
|_ _||Zp 4 - Loosen handbrakes|
|ooo ooo ooo||Zp 5 - Emergency. Brake immediately.|
|_ o _||Zp 11 - Come. Used to call out for train staff.|
Train horns are sounded where a whistle post (marked with the letter "S" for "siffler") is present. If the whistle post is labelled "J" (meaning "jour"), the horn is only to be sounded between 0700 and 2000. Horns must also be sounded when passing an oncoming train, and shortly before reaching the last car of the train.
'o' : denotes a short blast on the horn.
'--' : denotes a comparatively long blast on the horn.
'-----' : denotes a longer blast on the horn.
'----------' : denotes a very long blast on the horn.
Code [ o ] - Before Starting:
- Indication to driver of the assisting engine that driver of leading engine is ready to start.
- Acknowledgement by the driver of the assisting engine.
- Engine ready to leave yard
- Engine ready to go to loco yard
- Light loco or shunter about to move
Code [ o ] - On the run:
- Assistance of other engine not required
- Acknowledgement by driver of the assisting engine
Code [ -- ]
- Normal departure from station on receipt of clear signal. This is usually followed by another long blast about 10–20 seconds after the first one, after the guard's all-right signal is received.
- Beginning of shunting operation (if shunted rake has passengers in it)
Code [ o o ]
- Call for guard's signal
- Signals not exchanged by guard
- Signals not exchanged by station staff
Code [ -- o ]
- Guard to release brakes
- Before starting engine from a midsection/station
- Main Line clear
Code [ o o o ]
- Guard to apply brakes
- Train out of control, guard to assist
Code [ o o -- ]
- Sudden loss of brake pressure or vacuum (perhaps by alarm chain being pulled)
Code [ o o o o ]
- Train cannot proceed on account of accident, failure or other cause
- Protect train in rear
Code [ -- -- o o ]
- Call for guard to come to engine
Code [ o -- o ]
- Token not received
- Token missed
- With wrong authority to proceed
- Passing stop signal at 'on' on proper authority
Code [ ----- ] - Before Starting
- Vacuum recreated on ghat section, remove sprags
- Passing automatic 'stop' signal at 'On'
Code [ ----- ] - On the run
- Acknowledgement of guards signal
Code [ ---------- ]
- Approaching level crossing or tunnel area
- Recall staff protecting train in rear
- Material train ready to leave
- Running through a station
- Approaching a stop signal at 'on'
- Detained at stop signal
- Crossing stop signal at 'on' after waiting the stipulated time.
Code [ -- o -- o ]
- Alarm chain pulled
- Insufficient vacuum in engine
- Guard applies vacuum brakes.
Code [ -- -- ]
- Raise Pantograph (electric loco only)
Code [ -- o -- ]
- Lower Pantograph (electric loco only)
Code [ o o o o o o o o o ] (Frequently)
- Apprehension of danger
- Danger signal to driver of an approaching train whose path is obstructed
- Moving in wrong direction on a double line.
- Also used by EMU motormen to warn passengers on a crowded platform of the approach of a fast train which will not stop at that station
UK diesel and electric locomotives are usually fitted with two-tone horns, sounded sequentially to distinguish them from the horns used on road vehicles, the tones being described as either 'high' or 'low'. In the past, both tones were routinely used. However, because of noise complaints, new rules were introduced in 2007:
- The introduction of a night time quiet period, between 2300 and 0700 when trains will no longer routinely sound their horns at whistle boards (they will always sound their horns when people are seen on the track). The night time quiet period was changed by Network Rail in 2016 to 0000 to 0600. 
- That where the technology is available, drivers should only use the low tone from the two tone horn at whistle boards.
- For all new or replacement train horns on trains capable of travelling up to 160 km/h a much lower minimum sound pressure level has been established – and a maximum sound level has been introduced (min 101 dB and max 106 dB).
British train Horns have two tones, high and low, and in some cases, a loud or soft setting. If the horn lacks a loud/soft setting then train drivers are to use the setting provided.
|Sequence||Loud or Soft||When horn is used|
|H/L||Loud||General warning to individuals on or about railway tracks.|
|H||Loud||Emergency warning to individuals who are on the track or workers who fail to acknowledge a train's presence (Sounded Repeatedly.).|
|L||Loud||Used at Whistle boards (Between 0700 and 2300).|
|L||Soft||Warning signal when operating in depots or on sidings.|
|H||Loud||Used for special/local signal.|
|H||Loud||Wrong-direction movements, against normal railway traffic flow (Sounded at frequent intervals.).|
Placement on locomotives
As many individuals do with their personal vehicles, railroads order locomotives with different options in order to suit their operating practices. Air horns are no exception, and railroad mechanical forces mount these on locomotives where they are deemed most effective at projecting sound, and for ease of maintenance.
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The following are samples of select air horns as used in North American railroad service:
AirChime, Ltd. traces their beginnings through the work of Robert Swanson in 1949. Prior to the early 1950s, locomotives were equipped with air horns that sounded a singular note.
Swanson sought to develop an air horn which would mimic the sound of a classic steam whistle. Using ancient Chinese musical theory, Swanson produced the six-note model 'H6'. This was impractical for railroad use, due to its relatively large size. Railroad equipment operates over routes restricted by loading gauge, a difference of only a few inches may prohibit that equipment from operating on the line in question.
Swanson would later refine his 'H6' into the model 'H5'. As the numeric designation indicates, the horn sounds a five-note chord.
In 1950, AirChime introduced the 'M' series, a further improvement on the earlier horns through elimination of unnecessary moving parts. Among the earliest customers of the AirChime 'M' was the Southern Railway, which sought replacement horns for their motive power. The company announced this program through the placement of a full-page advertisement in the May 25, 1951 edition of the Washington Times-Herald.
Under Swanson's guidance, AirChime would focus on ease of mass production, low maintenance, and reliability in their air horn design, with the development of the 'P' (1953), and 'K' (1954) series
Buell Air Horns
Founded in 1912 as The American Strombos Co. of Philadelphia, PA, Buell sold modified marine horns for rail use. They were often installed on small locomotives, electric interurban equipment. and railcars (for example the Doodlebugs).
Buell has recently made available a line of air horns specific for railroad equipment.
Gustin Bacon Mfg. Co.
Leslie Controls, Inc.
Leslie Controls, Inc. (originally the Leslie Company of Lyndhurst, New Jersey, later Parsippany, finally relocating to Tampa, Florida in 1985) began horn production by obtaining the rights to manufacture the Kockums Mekaniska Verkstad product line of "Tyfon" brand airhorns, marketing these for railroad use beginning in the 1930s. Their model A200 series would later grace the rooftops of countless locomotives, such as the legendary Pennsylvania Railroad GG1, as well as thousands of EMD E and F-units. Leslie eventually introduced their own line of multi-note airhorns, known as the "Chime-Tone" series, in direct competition with AirChime.
Poor sales of the Chime-Tones (due to the horns requiring an ample volume of air) led the Leslie Company to introduce a new line of air horns utilizing interchangeable components while using less air to produce greater sound volume than the earlier "Tyfon" series. Developed by Kockums, this horn utilized a back-pressure power chamber design in order to enhance diaphragm oscillation. Known as the "SuperTyfon" series, these horns would eventually supplant the "Tyfon" in railroad service.
Leslie Controls continues to manufacture "SuperTyfon" air horns for the railroad industry.
Prime Manufacturing, Inc.
Prime Manufacturing, Inc. had produced locomotive appliances for many years prior to their entry into the air horn market in 1972. Their line of "Pneumatic Horns" was basically a derivative of the Leslie SuperTyfon design (having taken advantage of a patent expiration at the time), though their horns employed heavier castings than equivalents from Leslie, and sounding a somewhat richer timbre as a result.
Sales were brisk (railroads such as Union Pacific and the Burlington Northern were notable customers) but ultimately disappointing. Finding themselves increasingly unable to compete in a niche market dominated by Leslie Controls and AirChime, Prime ceased air horn production c. 1999.
Westinghouse Air Brake Co.
Westinghouse Air Brake Company (known throughout the 19th and 20th Century as WABCO) was the first to offer air horns specifically for use with railroad equipment, as early as the 1910s. Their model E2 was recognized by many for the deep, commanding tone it produced.
In response to the Leslie multi-note "Chime-Tone" series, Westinghouse offered a bracket to which three of their single-note "honkers" could be bolted onto, achieving the same result as what the Chime-Tones did for Leslie.
Overshadowed later on by Leslie and AirChime, WABCO eventually ceased production of most horns for the North American market.
Railways in Australia often utilize the same type of air horns as their North American counterparts. Examples from both the Leslie Co. and AirChime Ltd. can be found there.
Notable horns used in Australian railways, sourced from the United States include the Leslie S5T, Leslie RS5T, Nathan Airchime P3, Nathan Airchime P5, and the unique Nathan Airchime K5UL-AU-LS.
In the earlier years of dieselisation in Australia, during the 1950s, the earliest diesel locomotives were fitted with two chime horns manufactured by the Westinghouse Air Brake Company in Australia. These two chime horns were the WABCO Pneuphonic AB, consisting of an 'A' single chime, and a 'B' single chime. When played together, the combination often sounded melodious due to the 'perfect 4th chord' produced when blowing in unison.
Throughout the early 1960s, the Roy-Victor-Butler (R.V.B) company of Spotswood, Victoria, manufactured and supplied the majority of horns fitted to the locomotives constructed from the late 1950s, to the late 1960s.
The R.V.B horns fitted to the majority of locomotives used in Victoria (Australia) during the late 1950s to 1960's, were the 3 chime models. They were not very effective and seldom operated as intended. Not to mention, they produced a very displeasing sound, eventually resulting in their nickname "screaming cats".
The company specialised in producing pneumatic tyre pressure gauge meters, tyre air pumps, valves, as well as small automotive klaxons. In fact, many vintage Holden and Ford vehicles may be found to have R.V.B electric snail horns fitted to them.
Three chime R.V.B horns have also been fitted to the older Electric Multiple Units (EMU's) operating in Victoria. The old Swing Door (train) and Tait (train) EMU's of the 1920s and 1930s, were fitted with R.V.B three chimes, while the Harris (train) (1950's & 60's), Hitachi (train) (1970's), and Comeng (train) (1980's) were also fitted with R.V.B three chime horns.
Many of the older Australian Commonwealth Railways GM class diesel locomotives were re-fitted with Leslie S5T's, as did the Commonwealth Railways CL class, with their horns re-located behind the pilot after their rebuild and consequent re-classing to CLP, and CLF.
A large order of Leslie S5T's took place in 1976 - 1977 in preparation for the Clyde Engineering built 'C' class diesel locomotives, which were constructed from 1977 - 1978. All the C classes were fitted with round-back Leslie S5T's as standard.
Through to the early to mid 1980's, with the Leslie company replacing their 'S' line of horns with the more improved 'RS' line of horns, the A class (rebuilt from B classes in 1983), N class, 1st & 2nd order G class and BL class diesel locomotives all received Leslie RS5T's as standard horns.
The RS5T's are somewhat more reliable than the previous S5T horns, however the sound produced by the RS5T tends to be flatter sounding, and harsher compared to the deeper and throatier sound emitted by the earlier S5T's.
Most of the X class diesel locomotives received Nathan Airchime P5 horns in the mid 1980s, to replace the wretched and unpopular fleet of R.V.B 3 chime horns originally mounted on the X class. Originally, it was thought of to replace the R.V.B's on the X classes, with the Leslie RS5T, however, this was deemed inappropriate due to the large size of the Leslie 5 chime horns, which would consequently reduce visibility significantly. Another factor was the Leslie horns were much dearer in price than the Nathan P5. And so the X classes received Nathan P5's, one mounted on the nose and one on the roof.
Third and fourth order G class diesel locomotives all received the identical Nathan P5's which the X classes received during the mid 1980s. This was to reduce costs, as the Leslie RS5T's were much dearer, as stated earlier.
The AN class diesel locomotive is thought to be the final class fitted with Leslie RS5T's in the factory.
The older S class diesel locomotives were fitted with R.V.B 3 chime horns originally. Only a very small amount still retain these. Most have been replaced with Nathan P5's. S307 and S317 were fitted with Leslie 5 chime horns. 307 was fitted with a Leslie RS5T, while 317 was fitted with a Leslie S5T. Both locomotives have air restricting components built in to the air feed to the horns, to reduce noise pollution.
Locomotives built from the mid 2000s to current, were fitted with Nathan Airchime K5UL-AU-LS five chime horns. The 'UL' in K5UL, denotes an 'Ultra-Low' manifold, which was fabricated in order for the horn to be as low as possible when mounted, in order to comply with the tight loading gauge in Australia. The AU, in K5UL-AU, denotes 'Australian market' as these horns were designed by Nathan Airchime in the United States specially for use in Australia. The 'LS' in K5UL-AU-LS, denotes 'Low-Sound' as these horns have multiple air inlets on the manifold, enabling low sound output in populated areas, as well as regular loud operation for general use.
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- See Hayes v. Union Pacific R. Co., 141 P.3d 1073, 143 Idaho 204 (2006). https://scholar.google.com/scholar_case?case=17886920228406310755&hl=en&as_sdt=2,5
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