Air Horns vs Steam Whistles

[rcdrye]

SP's GS-class engines had air horns as well as whistles.

[Jones1945]

Milwaukee Road Class A, a preferred engine for Hiawatha express passenger trains used horn 

https://en.wikipedia.org/wiki/Milwaukee_Road_class_A

PRR S1 #6100, one of the largest steam locomotive ever built used horn as well.

https://en.wikipedia.org/wiki/Pennsylvania_Railroad_class_S1

[Overmod]

Detailed thread on one of the Trains forums last year, involving the specific use of air horns on steam locomotives.  (See "RE: Air Horns and Whistles" in the 'Prototype' section of the MR site, and Milwaukee Road Streamlined Hudsons in Steam & Preservation around the spring of 2017, to start)

Some of the 'early' use of horns on locomotives was to make them sound more like conventional diesel 'streamliners'.  But there were very significant reasons why horns were preferable to whistles from an economy standpoint, particularly as water treatment and deoxygenation became important in practical locomotive performance and maintenance.

DL&W was using chime horns on many classes of power in the early 1930s, and there is now a clip on YouTube of one of the latter 4-8-4s being tested that clearly has the individual horn notes.

NYC made a substantial investment in horns across several classes starting about 1947, while retaining whistles (and, I think, long-bell whistles suitable for operation on superheated steam.)  Interestingly enough, there is no mention of horns on the C1a proposal (April 1945) even though this was intended for faster operation than the 'upgraded' 79"-drivered Niagara which prominently had the horn for regular work, probably by the time of Kiefer's comparative tests with E7s.

[ScottK]

Doing a research project of sorts, and figured this might be a good place to ask a couple questions about the transition from steam whistles to air horns on trains.

• When (approx what years) did air horns begin replacing steam whistles?
• How did the transition from whistle to horn occur? Were steam trains ever outfitted with air horns? Or did steam trains and whistles remain a package deal that were phased out together?

Any help with these questions, or even a point in a good direction, would be much appreciated.

Cheers