Here's the section of vehicle code that refers to horns.
(a) Every motor vehicle when operated upon a highway shall be equipped with a horn in good working order and capable of emitting sound audible under normal conditions from a distance of not less than 200 feet, but no horn or other warning device shall emit an unreasonable loud or harsh sound or a whistle. The driver of a motor vehicle shall when reasonably necessary to insure safe operation give audible warning with his horn but shall not otherwise use such horn when upon a highway.
I would imagine a train horn would be considered unreasonably loud.
It is definitely not illegal to simply put train horns on your truck.
Using the horns without discretion can, however, get you in all kinds of trouble, even though there aren't any specific laws regarding decibel level, etc. It's up to your local police officer to decide what's reasonable use of a horn and what isn't. If you pull up to an old folks home at 3am and start blowing a train horn, you're going to risk getting arrested, cited, sued, charged with assault, disturbing the peace... any or all of the above. Ditto if you blow your horn at a railroad crossing and cause panic or accidents. Blowing excessively loud horns anywhere that is too close to pedestrians (so as to cause distress or hearing loss) could also get you charged with assault or you might end up with a civil suit against you.
If you want a horn to use at customized truck shows, or to blow in remote locations for your own amusement, that's fine. But I would not advise the use of a real train horn in place of your regular road horn, or for the purpose of harassing people like you may have seen in the popular hornblasters video. One of these days those guys are going to get shot at or followed, dragged from their vehicle and beaten. I seriously hope they keep the camera rolling!
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