A very complete guide available online and served as one of the sources of reference for this page: http://atsf.railfan.net/airhorns/index.html

- Horns -

The first thing to know is that this is a guide to locomotive horns, not locomotive whistles.  Whistles were used on steam engines, but on modern trains, a "whistle" is the sound created by blowing the horn.

Secondly, horns aren't classified according to the railroad on which they're used, nor according to locomotive builder.  There are only two major airhorn manufacturers from which a variety of different horn models have been produced: Leslie and Nathan/Airchime.

Check the sections below for more information.

Nathan/Airchime Horns
M3 series
M5 series
P3 series
P5 series
K3 series
K5 series

Leslie Horns
S-3L series
S-3K series
More to be uploaded

- Horn Valves -

Older locomotives often have manual air valves where the engineer can control the amount of air going to the horn, which generally means softer-starting horn blasts.  It also means an engineer can blow the horn at less than full volume.  Many newer locomotives have automatic button-actuated valves that give the horn immediate full air pressure, with resulting immediate sound--no half-measures.  The result is a difference in the character of the horn--compare the two recordings below of the same horn type (Nathan K5LA) with different valves.

SLR RM-1 #805 (June, 06, 2005) - manual valve
LLPX GP38-2 #2235 (June 08, 2001) - automatic valve

- Horn Pitch and Echoes -

The Doppler effect causes a horn to sound higher-pitched that its actual sound when approaching, and lower-pitched when receding.  However, the echo of a horn blast is not always affected by the Doppler effect.  As a result, the echo will generally sound lower-pitched than the horn blast on an approaching train, and higher-pitched than the horn blast on a receding train.

CN train (1993) - train approaching
LLPX GP40 #3207 (August 26, 2002) - train receding

Michael Eby 2007