I just finished installing my air horns yesterday, after two months of fiddling with various setups (i wanted something cheap and didn't want to use a compressor).
I'll take some pics and hopefully a video tonight, but here's what I basically used:
- 5 gal hyundai air tank with tire valve on intake and 1/4 npt on output. $30 at Walmart (Canada). Takes up to 100 psi. I fill it up from my tire compressor.
- Wolo 402 horns. They're small and cheap ($30) but they sound pretty good when powered by the 80 PSI in my air tank.

They come with a tiny 10PSI compressor that makes them sound squeaky as hell. Threw that away the day I received it, lol.
They're nowhere near real train horns though. I plan to upgrade to the big 3-horn Wolo "Siberian Express" (~ $100 on amazon). Those are huge and sound badass!! I think they're Nathan K3LA clones.
- $30 12V pneumatic valve with 1/4 npt on both ends. I've plumbed it directly to the output of the air tank, didn't want to use any air lines in between to avoid leaks.
- $5 Wolo air horn button (amazon).
I installed the air tank in the cargo area on the right, next to the 12V outlet, and routed the hose through the spare tire access hole under the floor.
Then i installed the horns under the truck, next to the spare tire (zip tied to the chassis).
I'm using the 12V outlet in the cargo area to power the valve. I used about 15ft of twin 22ga wire from the valve to the horn button in the front. It was really easy to hide the wire under the trim. I placed the horn button to the left of the steering wheel, right next to the power pedal button.
Pics tonight. I didn't have a chance to actually blow the horns while driving yet. Just tested them in the garage and then tested only the air setup on the truck (without the horns). I reckon they output about 125 dB, so I won't be abusing them (the legal limit over here is 100dB), that's why I didn't get a compressor. A 5gal air fill up should last me a few good months.