One of the most important parts of the entire system is the delivery valve,....make sure it is a true 1/2" valve.
I have Grover Triple air/train horns, a 115volt air compressor filling 15 gallons worth of tanks...(sound clip is at bottom of this reply)
I wired it directly into the stock horn switch(steering wheel).
Here's my set-up:
Horns mounted in the custom Safari rack I fabricated.
I have the 115volt air compressor, air tanks, and everything else mounted inside this tool box.....the 10,000 watt inverter that powers the air compressor is mounted above the wheel well bump in the bed.
and because I SUPPER SIZED my set-up(not 12volt).....I can also run HEAVY DUTY air tools.
and also,...because I have the monster inverter....I can run ANYTHING 115v out of the back of my truck
BTW......I also hooked my air horns into my alarm system.
here's the vid.
YouTube - Tanks Train Horns!