Years ago an old time SP engineer explained the use of horns verses whistles. SP hired an efficiency expert to reduce costs. This was a big deal right after WW2, efficiency experts were the buzz word of the day. I don't remember the numbers now, too long ago. It was determined the cost of blowing a steam whistle was several times the cost of blowing an air horn. On air horn locomotives the horn was to be used at crossings. All train movements were done with the whistle.
I grew up near the switch at Springfield Junction, this was the North, RR east, switch where the siskiyou line joined the Natron Cutoff. I was fortunate enough to hear lots of whistles for train movements and air horns for two crossings just RR W. I remember one morning hearing a series of short sharp horn blasts with a few whistles mixed in followed by a big bang. The morning Cascade hit a semi trailer loaded with lettuce. Personally, I like the sound of the SP steam locomotive air horn. You can still hear whistles but never the air horn.
Jim