I was watching the latest Trains DVD, the EMD Geeps. The Deleware & Lackawanna 4118, was doing some switching. The engineer signaled with the air horn. Not a horn but, a steam whistle. Was this the factory installation, or an after market installation. That is the best part of the DVD.
I grew up along the Minneapolis Northfield and Southern, their diesels used Hancock Air Whistles. I imagine they could be fitted at the builder, or the railroad could add them later. They basically work like an air horn but sound like a steam engine whistle. It was quite a downer when the SOO took over and started to BLAT the neighborhood with airhorns!!
Anyway, I believe a couple of big railroads (Penn Central?? New Haven??) used them in the past. Progressive Rail (that now operates much of the old MN&S with diesels using a similar paint scheme) have some engines with Hancocks, but they aren’t very loud compared to air horns so they generally use the air horns for grade crossings.
After the start of AMTRAK southern RR equipped a couple of its E’s with steam whistles. I only heard it once and it was scary because you know what I was wondering. “back to the past”? Believe it was eng 6901.