http://www.picbasic.co.uk/forum/showthread.php?t=4751
Putting DCC in the search field gets you a few results (I know, DCC is too short for the regular search)
Not a new idea though, putting an AC signal on a DC rider. I would think the hardest thing (if it's really that hard) is figuring out the hardware to superimpose this AC signal onto this DC carrier. Figure that out, and it would be just like sending serial data over 2 wires.