Doing a bit of searching DCC seems popular for model trains, where a squarewave is applied to the track, and the ac cycle lo/hi width modulated to provide data, presumably manchester encoded to keep the average dc zero so you can use standard motorised trains.
An interesting idea and I'll remember that for future ref, the current modulated rs232 system will do fine for my needs and needs little code or hardware.
I've installed siemens asi bus safety network systems on some of the machines I maintain, you can put severla stop buttons, light curatins and devices all on the same net which comprises of just 2 wires, that probably uses a similar system, only one thats certified to the required silly levels for safety gear.