Hah, putting poles on both sides of the rail works perfectly.
I just add one to each side just after the rails merge to the single lane and then the tap point gets added just before it splits into the loop.
That's not even something I would call a bug... hell, you could add it as a feature and say the towers can only power the rail on it's own side and so you need one for both sides for it to work
If you're leveraging the built in signal network to do this and not making up additional code to create a new network type, I can only imagine that the current method is going to be the lightest on UPS and will probably have the least chance at adding in incompatibilites with other mods too