Jon,
I brought my Cab 1 off the shelf, disconnected the legacy system, and installed the old TMCC system. I got the same result. Once I set the ID, the car work as it should (albeit limited functionality because TMCC), but whenever I remove the power from the track and then restore power, the car has forgotten its ID. It won't even respond to a "factory ID" like 1.
Thanks for your thoughts!