Very simple signalling over 12V DC power wire?
One of the comments mentioned DCC, but since you want to keep it simple (i.e. understandable for you specifically), you might just have a look at DCC's physical design and invent your own from there.

Basically, all it's doing is providing constant power at some DC voltage and using the polarity of that voltage to mean something.  So the circuit looks something like this:







simulate this circuit &ndash; Schematic created using CircuitLab

The transmitter's buffer and inverter need to be strong enough to power all the receivers, so I'd suggest making at least the final drive stage out of discrete transistors that are designed for that job.  Search google images for "cmos inverter" to get a basic idea.  You'll need one P-channel MOSFET to pull up and one N-channel MOSFET to pull down.  With 2 lines to drive, that's 4 transistors total.  When choosing FETs, pay special attention to switching times (shorter is better) and Rds_on (smaller is better).  When your prototype works, keep a finger on the transistors.  If they get too hot to hang onto, they need a heatsink.

The receiver's power supply is identical to a typical AC-in DC-out supply because it needs to accept either polarity.  The signal is recovered by actually measuring the incoming polarity.

The diodes will drop some voltage, so if you really care about the final output, you'll need to use that much more at the transmitter end.