I've actually thought about that, but discarded the idea. I think this is calling for disaster because signal wires are more fragile than electric wires. The worst thing that can happen if an electric wire is connected to the wrong pole is that two networks may be connected for a moment. That's negligible, but doing the same thing with signal wires may taint the signals of another circuit network, and debugging that would be hell!
There's another reason: Powered rails are just rails with an invisible electric pole on top. To be able to conduct power, it must be able to connect to rails going left/straight/right on the front and on the back. We would need 6 connections for that, but each pole may only carry 5 wires. (Actually, I haven't succeeded yet in building a track where all 6 connections would be needed.) If I'd add signal wires, one pole wouldn't be enough to support all connections. Perhaps it would be possible to use 3 different poles per rail track (one for each wire type), but that would hurt UPS.
Also, the power-to-rail connectors aren't too good at the moment. It would look stupid if they were to connect to hidden poles (may be OK for straight rails, but not for curves or diagonal rails as the poles are placed beside them). But if you place a connector in reach of two or more networks, you can't see where the connection comes from. That would make it easy to connect to the wrong rails, or even to connect two networks that were meant to be separate.
I agree that the powered rails are useless if you want to connect outposts via the circuit network, but adding signal wires doesn't seem such a good idea if all consequences are considered.
