First let me tell you that the horns you hear at the crossing are not running full blast, and these things are loud, very loud.
Unless you have a way to regulate the air, a 3 chime horn at street level blown at full volume will cause damage to anyone near or in front of it, and I doubt you want to go to the expense of mounting a horn, compressor and valve, plus a limiting regulator.
We have a jerk down here who has mounted a 3 chime in his pick up, he delights in driving through parking lots and sounding the horn whenever he gets the chance, and I have personally seen his scare a lady so badly she had a wreck.
She was backing out of a parking slot; he was traveling down the lane, then stopped, and waited until she was most of the way out of the slot and blasted her with it full bore….she was so scared she floored the gas and slammed into the cars across and behind her, destroyed her car and heavily damaged two others…he thought it was hilarious until the cops showed up, they ticketed him for excessive noise and aggressive driving, and on the accident report, they listed his horn and his actions as the cause of the accident.
I am pretty sure his insurance company found nothing funny about it at all.
Trust me, train air horns make the ones on 18 wheelers sound like bicycle horns, and I stress again, they can cause physical damage at full volume and at street level.
As advised, email J.C Whitney for their catalog and look at the quieter and cheaper alternatives, and think before using it.
No one expects a sound like that behind them on the freeway, and scared people do stupid things.