I have 2 sets of train horns on my superduty. 15 gallons of air goes pretty quick.
I mounted one to the skid plate for the transfer case & the other up in the bedside on the passenger side forward of the rear wheel.

I have 2 real, authentic, genuine tornado sirens if a guy wants to go that route. $5k each. You need 480v 3phase to run one.

for train horns the trick is to use the largest ID hose possible. the larger train horns use either 5/8 or 1/2 inch ID hose. also you need a large volume tank, PSI is important but you need about a 2500 cubic in tank, Volume is just as important. On my last truck I had real train horns that were fed by two tanks that were 4 feet long and 8 inches wide. Also if you use a valve to turn them on instead of an electric solinoid you can vary the sound just like a train

Nathan K5LA . Period.

Go K3 of you dont have the space or budget for the K5.

Dont waste your time on the hadleys or any of the other over the road truck stlye horns.

If you want loud, and the real deal. Go with a true Cast Locomotive horn.

Nathan and Leslie are the real deal

http://www.youtube.com/watch?v=XvdjO...layer_embedded

watch this video...I pissed my pants I laffed so hard