The air horns on those SP steam engines were single tone and faced forward from the front of the locomotive, thus were pretty useless for signalling any crewmembers to the rear, especially a flagman.

Though I have to admit that my specific memories of that era are a little less than perfect now, as a 12- and 13-year old I got to know many of the crews that worked through Hayward and on the then steam-only (Consolidations and the occasional Mogul) Hayward local. I got into the cabs of a number of Daylights and articulateds (even rode several miles on the 4179) and my recollection is that those airhorns were operated by squeezing a handle (valve) that sat at about waist height to the left of the engineer's position. For some reason I also think that possibly the whistle, at least on the Daylights, was also operated the same way...two handles side by side. I may be wrong on that but...? Anyone else recall? Maybe 4449's current setup would clear up that question.