Yes, great advice from John! I appreciate everyone's input on this!
And...the problem was: TWO bad sensors packaged with the first IRV2! Both were in new packaging, and I handled them carefully as I installed them. As improbable as a double failure is, that's what it was.
I'll try to get an RA from Lionel to have them replaced.
I have a related question. Does anyone know of a source for just the sensors? The IRV2s support 4, but come with only 2. I have been unable to locate them for purchase.
Thanks again everyone!