OK, so it looks like it was Possible Diagnosis #2 from my earlier note -- -- "The LCRU1 board failed in a way that it's still communicating with the RS2.5 board, but doesn't respond to any of the TMCC commands."
From what you've told me, your RailSounds board is acting like it should without the purple serial line attached to it, which is good.
There's really not much else that can be done, especially remotely. We know the LCRU1 is getting power because it's definitely sending the signal to prevent the RS2.5 board from starting when the serial line is attached. You've already pulled and re-seated the processor. I think you've confirmed that you've not had any lighting or movement throughout this whole process. Lastly, there's no way to perform a software reset/restore on those boards -- that feature started with the LCRU2.
Therefore, I'd recommend replacing the LCRU1 board.
TRW
