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.
Our community is FREE to join. To participate you must either login or register for an account.