I"ll check on the signal levels when I am back up at my layout location. It's 35 miles away up in the mountains where I used to live. In looking into signal conditioners, I believe I did try that a few years back at the same time I tried the isolation transformer.
The DC power approach is interesting, All my engines are MTH of PS2.0 (5V) or later. So they can all run with DC track power as well as DC input power to the TIU? I assume that is what you get to the track if you put DC into the TIU.
Thanks everyone for the suggestions!