After much searching I have finally made the mod making my train horn work with the stock horn switch. I used a SPDT switch and a 12v relay, I found that it was easier then I thought it would be. I will try to get pics of it later.
You’re welcome
congratz!
Another simple way would have been to run wire direct from the stock horn to a switch on the dash and then to the air valve. Thus when you sound the stock horn, if the switch is “on” the train horn will sound.