A few years ago there was a phenomenon in the early days of YouTube where pranksters would fit huge 140 decibel train horns to their cars to scare members of the public. I decided I wanted to try it, because I'm a manchild.
Step one was acquiring the necessary parts. I needed a large, robust air tank taken from a DAF truck and mounted on a large piece of MDF in the boot.
The image at the bottom gives an idea of the setup (apologies for the low resolution, it was filmed when phone cameras were potatoes)
Next I needed a compressor that could power the tank to around 250psi (17 bar)
A few gauges and some wiring were the easy parts, acquiring an actual train horn is the hard part. Thankfully Arriva Trains Wales were scrapping their fleet of ageing Pacer trains (based on old buses!) so I was able to find one at a decent price. All in the setup cost around £400 and it was LOUD! The compressor took a minute or so to get the pressure up, then the two-tone switch let rip the aural chaos. The reason I ended up selling the horn was due to the sheer damage it did to my ears each time I used it.
The most fun was to be had at train level crossings. A quick blast on a green light and the traffic all comes to a stop, thinking a train is about to come tearing through. Childish stuff, but this was when I was in my early twenties. I eventually sold the setup to a clown, to use on his clown car. Video above (the noise doesn't do it justice) and was filmed on an old camera, hence the quality.