Don’t do a train horn. As someone with ptsd and a heart condition, those things are an absolute menace.

Not to be a bummer, but the loudness of the horn will be limited by the max amperage of the relays. If the horn relay maxes out at 15 amps for 12 volts, you get about 180 watts of energy to the horn. Most horns are just a coil that vibrates at a set frequency to move the rubber or vinyl diaphragm inside the horn. That’s the noise.

Short version, you can put really any horn in there, but the total amount of watts will be the limiting factor. A more efficient horn (aka a new one) will make more sound for equal input wattage.

The other option is to make an array of junkyard horns and set them up with more relays to lift the limit. You’d need to have multiple fused connections unless you just want to be a hillbilly and run them right off the battery with a giant ass relay.

You might start a fire but it will be loud as shit for a few minutes before it burns down.