Take a look at this write-up by Azatrax. A relay is used to apply power to either the proceed/vertical/green light or the stop/horizontal/red light on the signal bridge. The relay is controlled by the insulated-rail of the Activator Pack.
http://www.azatrax.com/block-detector-3rail.html
The Azatrax relay module runs about $10 (plus shipping). If DIY component-level assembly and soldering is in your comfort-zone, you can buy parts from eBay and make the equivalent for less than half that. There are OGR threads showing how to do this which can be dug up if you're interested. But most guys understandably want a plug-and-play solution...