Atlas seemed to be able to light the domes on the Zephyr cars,how much can it cost,plus wrong end on observation.
Atlas used a pair of chip LEDs on the top side of the light board to illuminate the dome. Its not a whole lot of light, but just enough to reflect off the windows and make it seem lit. Cant imagine it cost more than what it costs to buy 2 LEDs, but that would require making 2 boards.... one for domes and one for everything else.
Drilling holes in the floor of the dome would work, provided that you didnt drill them directly above the circuit board for the lights downstairs. I also bet the board would support an extra LED if you soldered some wires on in parallel to the other LEDs in the board.