Sunday, 25 March 2012

The Air Horns of the Class M2 Locomotives

The Air Horns of the Class M2 Locomotives
M2 icon
Gyan Fernando

One of the distinctive features of the General Motors EMD G12 Class M2 locomotives of Sri Lanka are their air horns. 
The triple air horns, which are mounted on top of the cab, are finely tuned with two of the “bells” pointing in one direction and the other in the opposite direction.
They are also controlled by a pull-cord, in traditional fashion.

AirChime Nathan 
The horns fitted on all the M2 locos are manufactured by AirChime and Nathan. AirChime and Nathan are now
570 Alberta showing air horns
 one company under the Micro Precision Group with the loco horn division based in British Columbia, Canada. 
On the Class M2 locomotives, air horns with either one, or both the company names, may be seen.
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 but a single note.
Swanson sought to develop an air horn which would mimic the sound of a classic steam whistle. 
AirChime air horn on Montreal
AirChime logo on 626 Montreal
Using ancient Chinese musical theory, Swanson produced the six-note model 'H6'. This was considered impractical for railroad use, due to its relatively large size.
AirChime air horn on Montreal
AirChime logo on 626 Montreal
In musical terms, the triple horns play a chord which is pleasing to the ear and this is what makes the Class M2 horns distinctive.

How Air Horns Work
Train  air horns are operated by compressed air, typically at 125-140 psi (8.6-9.6 bar), and fed from a locomotive main air reservoir.
Newfoundland with Nathan logo
595 Newfoundland with Nathan logo
The flow of air through the horn produces an action known as oscillation.
Oscillation in a train horn is accomplished via a diaphragm assembly enclosed within the power chamber.
When air is applied to the horn, the diaphragm vibrates against a nozzle. The oscillation of the diaphragm against the nozzle produces sound.
The “bell” or the trumpet, modulates the sound and determines the pitch. In general terms, three horns sounded together produce a musical chord.
Good quality Airhorn bells are usually sand cast in aluminium. This  probably explains the lack of corrosion on the M2 loco horns.
How air horns work
How airhorns work (Wikipedia)

Logo from pic above
The Human Input
The M2 horns are worked by means of a lanyard or cord instead of a button, and this allows the engineer a degree of flexibility unlike with horns operated by a button. 
For instance, the engineer can “feather” the horn by varying the pull on the lanyard. In musical terms this is known as a "trill". 
This is not possible on button operated horns.

Footnote: Astute readers will note that the Nathan logo on the air horn of Newfoundland is upside down indicating that the bell has been fitted to the chamber the wrong way. In the picture showing the detail of the logo (above) I cut out the logo and inverted it

Click on the images for larger versions. All the photos on this post belong to the author. The other illustrative material is from copyright free sources.

References and further reading:
Video Links on You Tube


  1. Great article Gyan. Very informative indeed. North American built locomotives are characterised by their triple and ocassionally five trumpet horns. (Leslie & Klaxon too make them). I distinctly remember when M2s 628 & 629 (originally the 'cement haulers') came around 1966 or so, they had a single trumpet horn much like a megaphone, fitted atop the cab door sides . Sometime 'down the track' they seem to have acquired Nathan triple horns. Keep up the good work!!

  2. Thank you for the great information. I have a great fascination with these Canadian engines and specially the amazing sounding horns which were on Montreal and Vancouver. Do you have any idea what notes they were tuned to? It is obvious they had the Canadian tuning but it would be great to find out their original tuned notes. Thank you again and keep up the great work.

  3. I am not very musically inclined so I cannot work out the chords! Will do my best!