As far as I can tell, if the OP (or his property) do not fall under the HOA, then the HOA has no say in the matter. I do live in a deed restricted area with an HOA, but I can still take actions against neighbors, personally. The HOA doesn't need to be involved unless they, as the HOA, choose to be.
That said, individuals can still take actions against one another on a personal basis. The people coming to complain about the horns could avail themselves of various city/county ordinances about "excessive noise," etc. There might also be ordinances about using sounds that might imitate a train and cause confusion among drivers, etc. But again, these would be individuals taking actions against other individuals.
As for the horns themselves, there are a ton of train enthusiasts out there that own train horns. Some of them are, in fact attached to their vehicles and are used in place of the standard vehicle horn (or they can switch back & forth). I'm amazed the OP didn't have a "Nathan K5LA" horn.
That's the standard horn used on most commuter trains (METRA in Ill., Amtrak, etc.). If I could afford one, I'd get one as well, but I would never use it near my house, I'd go way out to the sticks to use it. I am aware of a local fire company that put a K5LA on one of their engines as a replacement for their standard air horn. Not sure if that's good or not as, again, it could confuse drivers who are stressed enough dealing with an emergency vehicle bearing down on them. I know about that stress. I drove a Type 3 ambulance for many years.
But no, I would not use it in a populated area. I know some of these horn owners that have them in their cars often like to prank people out on walks, etc. Not good. If they did it to a LEO on patrol, I think they'd be in a bit of trouble. Just as bad as discharging firearms in a populated area (in my county you must be at least one mile away from the nearest neighbor before you can have an outdoor range, etc.).
