Is it possible you may have the red & black wires crossed at the track? Not likely if the engine remains dark and silent but apparently with proto-3 it doesn't matter. However it does matter with proto-2 (red to center rail)
We know the read finds the tiu... so the remote and tiu are communicating.
You really can't check out the dcs signal if you can't add an engine.... I'm guessing this may be something simple we've overlooked.