The Hancock 4700 and 4710 air whistles are not necessarily the panacea that some believe them to be, having compromises baked into the design... the biggest being the 1/2" NPT air supply that feeds it. Dirt, oil and moisture in the air supply doesn't help things. By the time they were marketed, nobody was plumbing locomotive horn lines from the cab with anything bigger, and consequently that annular air slot is pretty narrow and the diameter of the bell relatively large for such little flow. I thought - and still think - that where the speeds are low, they were not a bad warning device, though they were anemic sounding compared to a 4" or larger steam whistle and have a slightly different, "breathy" timbre.

I have owned a number of them over the years but they are now collectibles for those with deeper pockets than mine, and they tend to deteriorate with long-term exposure to damp/corrosive environments- they are an eclectic mix of aluminum, bronze and ferrous bits that make for a dandy "battery" in the presence of an electrolyte.

IIRC, Rizzoli sort of "re-created" one a few years back which I believe turned out all right in the end. There is no reason why someone with a machine shop couldn't fabricate a 4" whistle optimized for use with a typical diesel-electric air system, though making a pretty one might be a chore. Personally, I think a 4710 (the type without the reflector) makes more sense, and it would be no harder to make than one of the current steam whistle reproductions. The Hancocks' nose caps might have been a nice aerodynamic touch, but they caused more problems than they solved and could profitably be left off.

But anyway...

I consider the easy button for mollifying NIMBY horn-o-phobes to be a Nathan Model P (the older the castings, the better, soundwise) three- or five-chime horn, "single diaphragmed" and with the right restrictor in the manifold's inlet. (The "right" restrictor varies with individual applications.) Control it with a Viloco modulating horn valve in the hand of someone willing to use finesse; unfortunately, nobody is currently making the rubber disks needed to recondition these valves, so refurbishing one can be a bit difficult.

JMHO

While in college in the 1960s, I worked on the now abandoned Narragansett Pier Railroad. The Pier's 65-ton Vulcan #40 had two "noise makers" on it: a single note honker; and a three-chime air whistle that came off a New Haven RR Alco S-1 switcher. The whistle was used in town and the horn out on the 7-mile line.

Thanks!