Kevin, I think I was editing my previous post while you were typing yours with the new pic. Sounds like we hit the same conclusion. (and I was apparently typing this one while Lou and Ted were typing theirs 
)
I'd review the hot/common connections. I generally also try to stick to black being the common for everything, but that is not electrically required, obviously.
The earlier picture showed the wires going under the track only under a tie. Were they under a tie where you found the problem, or just under rails where there is no tie? If under a tie, it's still a mystery how it contacted the center rail, unless you do have hot to the outside rails.
Also, just a nit, but you have a switch machine in the last pic. You didn't mention that before. Even with everything off the track, the switch machine can be a source of issues as well, since it presents a load as well and the bulb and solenoid is still there even if you remove everything else. Is the switch powered off of the accessories run via a constant voltage plug, or is it powered with track power?
Also, re: Lou's comment on multiple feeds, I noticed on your earlier pic you had 2 sets of ring terminals for D, but only one for A. Did some previous situation cause you to do that, what is the background there? Bigger loop (more track) for D?
-Dave
