I am not familiar with the 2404 cars. Are those streamlined or heavyweights?
The aluminum cars have separate window strips that can be changed. Some of the Williams streamlined plastic 18 inch cars are made of clear plastic and painted, with the windows masked off. At least the vista dome is like that.
If you just want more light coming through, why not put in lower voltage bulbs? Bulb number 51 is 6-8 volts. If you are running Williams engines in conventional mode (not TMCC or command, or DCS) then you are probably running around 7 volts on the track.