I’d go back to Norton’s original idea of taking off a body shell and looking at one of the car’s LED boards to see if there is any obvious sign of damage to the components on the board connected to track power - maybe the caps have blown or some other component failed.
The Lionel replacement parts illustrations are not clear enough to ID individual components but, from memory, my boards have a set of conventional components to convert AC to DC with a pair of caps to supply power to the LEDs. But even to my untrained eye these looked odd compared, for example, to 3rd Rail’s old LED strips or GRJ’s LED passenger car lighting sets.
I’m not techno-savvy enough to guess what sort of power incident could have caused similar components in each car to fail at the same time and I won't speculate on that.
