RS-1 15 is correct; I was advised several years ago, by a gent that has signed many a cab card, that the Hancock 4700 is not generally loud enough to satisfy FRA requirements.
Aside from the lack of dB, do you think that any of today's cellphone- using, sound system-blasting drivers are going to associate the sound of a device designed to sound like a steam locomotive whistle with a present day train, even at a grade crossing? I imagine not-
They sound great, when supplied with enough air volume, (more than most air horns), but for use at public crossings, there is just too much liability involved in using one.