Swapping Boards issue

Did you do a very complete visual inspection of the initially bad locomotive wiring and the motherboard?  Since the symptoms suggest you may have killed one of the working boards in the problem locomotive, that's where I'd start.

Are you SURE that you haven't done anything else that might affect running?  FWIW, it's best to swap boards one at a time so you don't end up with exactly the situation you now find yourself in.

What I'd do is get someone to test all the boards and find out which, if any, are bad.  On the surface, it sounds like something is taking out a board in the one locomotive.

There are no boards in B. B was a TMCC equipped unit at one point but runs a conventional unit. I recently found A as a working unit, so having 3 working boards from A should make B work.

I guess a more complete description of what process you're following would be useful.  From the sound of it, you obtained B and tried to upgrade it to TMCC with the A unit boards.  Clearly, there seems to be something different about the B unit.

We are all good. It seems that leaving these things sit for a while and coming back to them makes a world of difference.  Both work, with the one set of boards. 

No  it was not an upgrade, B had TMCC all over the box and the instructions were all there. It was a reduced item that I bought years ago but never did anything with. Now that I found the second unit I could compare what was there and see what was need.

thanks for your help.

Sometimes walking away and coming back is the silver bullet, glad it all worked out.