What was the 12v AC socket for on the 7864 transformer? The 7864 transformer had, in addition of the main regulated outlet for controlling trains and a fixed 12v DC for plugging in remote controls and lights, a third socket available which provided 12v AC.

This third socket was blocked with a plastic plug, as seen on about every photo of it I can find, and I've never seen any reference of it being used in an official, LEGO-endorsed way. Even the official instructions bundled with it don't mention the port, treating as if it doesn't even exist at all.

So I wonder, what's the story behind this? Did LEGO have something else in mind with it, which was later abandoned? Or did they just toss it in because they had it internally available?

Retro-compatibility could also have been an answer, but the two AC socket on the blue transformer are apparently 16v (for switches) and 3,8v (for lights), so I'm thinking that's not it either.
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