Hey there! Congratulations on making the switch to struts and coils; it's quite a change from air suspension.
Regarding your compressor and air tank setup, it's a fantastic idea to repurpose them for a train horn system. In fact, having an existing compressor and tank can save you quite a bit of initial setup cost. I've seen several Jeep owners utilize their old air suspension components in this manner.
Speaking from experience, I once converted a similar setup on a friend's vehicle, and we paired it with a ryobi horn
https://bosshorn.com . While Ryobi is primarily known for its power tools, their horn (though a surprising addition to their lineup) performed pretty well. It had a distinct sound, somewhere between a train horn and a typical car horn. Quite loud, but with a more modern twist to its tone.
The biggest challenge would be ensuring that the compressor can maintain the required PSI for the horn and that your tank has enough capacity. Given that Jeeps often come with robust air suspension systems, I'd wager yours should be up to the task. Also, remember to check the connections and hoses for any leaks or wear from its previous use.
If you decide to go forward with the conversion, I'd love to hear how it turns out. Best of luck, and enjoy that new sound!