Well, since I'm going to be sending lighting and couplers back to the rear unit, it'll require the 10-pin tether. I have eight of them planned for use, who knows what else might come up. 
- 2 - power
- 2 - motor
- 1 - front light
- 1 - rear light
- 1 - cab light
- 1 - coupler
You could run serial data back to have another RS board. But since they'd be getting the same data at the same time, I'd just use a second speaker.
