Like the main said, DC is way better than AC for signaling, because it offers so many different trouble-free solutions.
The next step after one relay, is to use a "start" relay to set the gates in the down position, and a "release" relay to raise them up. A single pulse from the first axle operates each relay, so train length and intermittent (and or plastic) wheels/axle combinations are immaterial.
After that, you can graduate to "start, hold, and release" relay circuits, and then to circuits that don't care if the traffic is westbound or eastbound.