Chris, at the risk of repeating what has already been stated -- I connect the wire from each isolated rail to one side of my relay coil, which becomes "common"/ground when a train bridges a "live" outside rail to the insulated outside rail. The other side of each relay coil is connected to a 12VAC transformer "hot" side and is always "on." So providing a ground energizes the coil.
ALL of my power supplies, both AC and DC, have their "commons" and "grounds/negatives" connected together. So every common/ground is the same as every other. All AC "hot" transformers must be "in phase" (you can Google that if you aren't familiar.) So it doesn't matter how much voltage is on the track center rail (in my case, constant 18VAC) -- I only care about getting a ground. Even if you are using conventional control and have zero volts on the track, your ground is the same -- it works whether you have any power to the center "hot" rail or not.
Simply bridging the ground from one rail to another, and using that to power an AC relay coil, has not caused any problems with my Legacy signals. I'm still a novice at all this, so it's possible that a relay coil could cause problems without a flyback diode or other inductive voltage suppression -- but I have not seen that problem, and most folks here are not driving coils directly, like I am. It's also possible that the time-delay-off relays that I'm using already have that protection built-in -- I have not looked closely at the circuit.
