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.
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.