I have a hard time, if you are upgrading the engine, why would you not go railsounds- thus compatible with LCRU2 serial data and a decent sound upgrade over Sounds of Static- I mean steam?
Your sounds of steam board- while electronic, cannot decode serial data.
I really would like to see a link to this "I have a circuit diagram from @gunrunnerjohn that utilizes diodes and a relay to produce a DC offset to activate the horn if a direct serial line connection is not doable."
The reason I am saying that is, it then is a circuit that can decode TMCC serial horn data, and then do something, which would imply a microcontroller.
So yes, I'm interested to see where this is going as maybe, I'm just not aware of this circuit you mentioned.
Again, don't take this the wrong way. Either you love the SOS board and want for some reason to keep that, or you just don't want to upgrade to railsounds- hey I get it, that cost (now roughly $129 for ERR) and Lionel pulling a whole bunch of those parts- upgrades are not as cheap and easy as they might have been in the recent past.
I just feel like rigging something to trigger SOS from an LCRU serial data stream, is just not really my cup of tea. But if there is a way- good to get it documented in a topic and that same circuit could have other uses.