OK, one last thought. Some of these cars (the dome cars) have separate light boards that all wired back to the main board on which the electrical components are mounted, as shown below, parts 10, 11 and 12:
A test I have done is to feed some low voltage battery power to one of the separate boards to see if the LEDs light up at all independently of the main power components. I used a depleted 9 volt battery and just made momentary contact as I do not know what the voltage rating of the LEDs is. In my case they lit up so I knew the problem was either in the components on the main board or further back where track voltage is supplied to them.
P.S. I had separated the single boards I tested from the main board and worked out the polarity of the contacts leading to its LEDs.
