Just as a follow-up, Lionel sent me two correct AC Regulators code S03 that matched the one in my loco that was not recognizing Legacy smoke commands. Tonight I replaced it and now my loco smokes like it should. So thanks to @gunrunnerjohn for pointing me to that assembly as the likely culprit - it was!
I wish i could repair my old one, but John suspects the likely failing part is the 8-pin surface-mount chip labelled (beneath the S00 label) "MQ4CE RMGKFm", which he says is a microprocessor with custom-flashed code. And by the way, I don't know if there is any significance to the fact that my old part had a "S00" sticker on the 8-pin chip (i.e. not "S03".) I looked at the new part that Lionel sent me and that chip has a sticker labelled "S03."
Alas, the 2N6073BG triac is easy to get and easy to replace -- so hoping against hope and with no reason to believe this would be the failing part, I did order a few. They bolt this thing to the frame so I'm assuming it can get hot. The next time an S03 regulator goes on one of my locos, I'll try swapping in my bad one with a fresh triac. Why not?
Anyway, I wanted to let everyone following this thread know that swapping in a new S03 AC regulator fixed the problem of my 6-11117 loco not recognizing Legacy smoke commands.
