I am definitely not an expert in this area, but I just installed some in an Athearn and used 1.5V 15mA 1.2mm Diameter bulbs from Miniatronics corp. I used the existing Ditch light housings, and just drilled the center out of them (they were just the plastic molded ones for asthetic effect and not intended for actual use) I am using a Digitrax DH163 decoder and put the lights in a (series?) with a 1/4 watt 825 ohm resistor. The lights are quite dim so I think that I will put a 750 ohm on instead. It isn't anything like an Atlas, but if you want, I could take some pics of it for you.
If you go with the fiber optics you could light both of the ditch lights with one bulb. However, if you want them to flash, you will need to install two seperate lights instead.
Freelancer