Darn. I saw headlight lenses on some of the above pics, but they weren't Lionel. The Lionel 520 didn't have headlights. I looked up the Kline 21268 on Lionel parts, and surprisingly it was there. Unfortunately the headlight lenses were not listed in the exploded parts diagram. Thwarted yet again.
The 520 was my first train set when I was 4 years old for Christmas 1957. Evidently my dad bought a 6025 Gulf tank car to make a longer train. I still have the set plus the tank car on the shelf and I still occasionally run it. He must have picked up a junk prewar tender for the whistle because he wired it under my little 3'x4' Christmas platform and it worked directly from the transformer with a Lionel button.
I acquired two 520 Box Cabs and applied the options already cited by others in both units: drilled-out the headlights and added lenses and bulbs, added horn and bell, and installed a GG1 sound chip with speaker in the "dummy" unit (the closest sound set I could find, although not "correct"). I created a task for the pair ... to pull a track cleaning car.
Fun to run ...
Mike Mottler LCCA 12394