What was the 12v AC socket for on the 7864 transformer?
I had one of those, and afaik it was just used for the lights.  I had like 2 light poles in my set and they where used for that.   lights is about the only thing that could take AC current instead of DC in lego.  Those weren't LED's at the time.  I just can't find any pics online from the ones I have(had...).

Edit: Found it, it was powering this light brick: http://guide.lugnet.com/set/1140