Why not talk to a PIC in the module using I2C?? After all, the track is two wires.

You could modulate this on top of a standing DC voltage then control a motor using hard coded addresses on the loco module PCB and an H bridge driven by the PWM from the PIC.

Just a thought. I tried it and it worked fine. Be careful of the need to filter the I2C signals HEAVILY but not so much that you lose them.