Thanks for sending the sound sample of the GS4 for comparison.
CF's had a whistle and horn. Installing a horn might be difficult to get the very low freq. sound. Sound reminds me of a fog horn, which probably isn't too far off what it was really used for, high mountains, valleys, etc. I'll go for a wistle first and horn a distant 2nd. Fog isn't much of a problem for me.
"Baldwin delivered the initial group of AC-12's, beginning with # 4275, to El Paso, Texas in the fall of 1943. The balance was received in March of 1944. Interestingly, the 4294 was the last new steam locomotive ever to be purchased by Southern Pacific.
Baldwin utilized the same newer semi-streamlined cab shape that was practically identical to some of the earlier cab forward classes. Many consider these to be the most attractive of all the AC series. The AC-12's were equipped with both the conventional steam whistle as well as a single note "fog" horn."
Each horn maker had their own distinct sound. If we can learn the cab forward's horn's make we might be able to get a sample of the makers sound. One source (or at least it's a place to start): Five Chime Consultants: Railroad Airhorn Guide; http://atsf.railfan.net/airhorns/index.html
My question is, how would you make a horn in small scale live steam? Larry Bangham scaled a locomotive whistle with his Resonator Whistle design. The same problem of scaling is there for a horn. Is it possible to make a Resonator Horn?