[jeffba]

while some numbskull s-l-o-o-o-l-y accelerates almost up to highway speed.

So basically you want a horn loud enough that you blast them into the next county. Easier to do back east.

Blasting a train horn from behind will cause a braking reflex from me. So when you blow you may also want to stomp on your disk brakes. I bet I am not alone in that reflex.
Also many states allow one to call and report aggressive drivers. A train horn may fall into that category.