now this is just preliminary with no asbuilt or verification of what the OEM horn circuit actually looks like but this is how i plan wiring mine in.
In lamens terms what will happen in the circuits current state is that when you press your momentary switch in the center of your stearing wheel aka (horn) the OEM horn will sound.
Now when you flip the Auxilary switch and you pull in the 2 relays R-T-H and R-T-C.
Relay R-T-H sole purpose is to open the OEM horn ciruit and provide a path for the Electronically actuated solenoid for the train horn circuit. So that when you press the Momentary switch in the stearing wheel it makes the solenoid release the compress air to the horns.
Relay R-T-C sole purpose is to control when your compressor (M) can run. So in essence the toggle switch has to be flipped for your compressor to run. Then wire the Electronic pressure switch to control the on and off function of the compressor when the need for more air is needed.
This will make it pretty covert.