It cannot be all that complicated. A little plastic man comes out of the little plastic house and turns a little plastic crank, and the gates come down. To make the go up is even easier: He just kicks a little plastic dog (pall) out of the way, and the gates go up.
If you do not have little plastic people, I guess you will have to find a different way to wire it.
Or simply build a SUBWAY layout. Subways (at least the real ones in NYC) do not have crossing gates.
ROAR
The Route of the Broadway Lion The Largest Subway Layout in North Dakota.
Here there be cats. LIONS with CAMERAS
I am working on the same thing. I have the crossing bell circuit working. I need to build a flasher circuit and train detection. I was going to use optical detectors, a photo transistor and an IR led. I planned to hide them inside those lineside aluminum painted electrical boxes that you see up and down the tracks on the prototype railroad. This way the operation of the crossing signals is completely independent of track power, track blocks, and it works on DC or DCC, no problem.
Sounds like you are thinking of using twin tee current detectors. They work by sensing the current drawn by the train. They will detect locomotives and lighted cars, but ordinary freight cars draw no current so the twin tee won't see them. To make such cars visible you connect a 10 K ohm resistor from the North wheel to the South wheel. I have a sizable collection of rolling stock and the thought of retrofitting them all with resistors and metal wheels is not attractive, hence the optical approach. Also, should you stop the train blocking the road, the Twin Tee will cease to see the train, since you have turned the current off. At least on a DC layout.
David Starr www.newsnorthwoods.blogspot.com