Train horn

File:Nathan M5.JPG

Train horns are made of multiple horn units called chimes which produce different notes; sounded together they make a chord. The Nathan model M5 pictured is a 5 chime horn.

A train horn is an air horn which serves as an audible warning device on diesel and electric locomotives. Its primary purpose is to alert persons and animals to the presence of a train so they can clear the tracks, especially when approaching a grade crossing. They are also used for acknowledging signals given by railroad employees (i.e. during switching operations).

File:LeslieRS3L.jpg

Leslie S3L locomotive horn, once the most common horn on American railroads

History and background

Railroad trains have a much greater mass and thus a longer stopping distance than road vehicles. They typically cannot stop in time to avoid hitting obstructions on their tracks. Trains do not stop at grade crossings, where roads cross tracks, but rely on 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 efficiently 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 so the air horn design was scaled up and modified for railroad use.

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.^[1] 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.^[2]

Recent years have seen an increase of horn theft from railroad property.^[3]

Operation

Diagram of a typical locomotive air horn power chamber, showing operation

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 metal 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.

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.

Current production locomotives from GE Transportation Systems and Electro-Motive Diesel have utilized a lever-actuated solenoid valve.

United States

File:LeslieA200.jpg

Leslie A200-156, a single chime horn used on locomotives in the early days of dieselization

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.^[4]^[5] 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.^[6] 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 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.^[7] The new ruling preempts any state or local laws regarding the use of the train horn at public crossings.^[8] 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.

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.

Rule		Sequence	Indication^[9]
14 (a)		o	Applying air brakes while standing.
14 (b)	*	– –	Proceeding. Releasing air brakes.
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.

Germany

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:^[10]

Sequence	Meaning
_	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.

France

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.

India

The IRFCA FAQ lists the following:^[11]

'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

United Kingdom

UK diesel and electric locomotives are usually fitted with two-tone horns and, in the past, both tones were routinely used. However, because of noise complaints, new rules were introduced in 2007:^[12]

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)
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)

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.

MBTAHorn2.jpg

Horn mounted on MBTA control car when in Push Pull Mode
Nathan K5LA mounted atop a San Diego Coaster Cab Car
BNSF 6186 GE ES44AC.jpg

Horn mounted in the middle section of the roof on a GE locomotive
Makrofon.jpg

Typical horn mounting location on European locomotives
152-079-0.jpg

Low horn mounting location on a European locomotive. The horn is visible above the right side buffer.

Audio samples

The following are samples of select air horns as used in North American railroad service:^[13]