I lived near the Hayward-San Lorenzo boundary in the mid-50's next to the SP line from Elmhurst (Oakland) to Niles Jct. through downtown Hayward, a freight-only section (what passenger trains were left by then ran on the "Mulford" line to the west). The Hayward line regularly saw 4400's (Daylights) and various "Malleys" (4100's and 4200's) as West Oakland was pretty much the last bastion for steam on the SP. Like the 4400's, the 4200's (or most of them) had air horns in addition to the steam whistle. I think the older 4100's only had a whistle.

Many engineers on engines equipped with air horns seemed to use mostly the air horn at grade crossings but whistles were often used and some would occasionally use both at the same crossing. Generally speaking fog was not an issue in that area and speeds were moderate, as I recall.