Your automotive relay works "best" when 12V DC is applied to its coil to turn it on. As drawn, when a train is on the insulated-section, you will be applying 18V AC (from your Powerhouse) to the relay coil. This is frowned upon.
Additionally, when a relay is "off" (no power to coil), the relay's COM is connected to NC. When relay is "on" (power to coil), the relay's COM is connected to NO. As drawn, the Red would be on when vacant, Green would be on when occupied. Hence my question.
You have several options. Here's my suggestion. Other's can advocate a different option.
Since it's a 12V DC relay, provide a 12V DC power source. By tying the "-" side of the 12V DC source to the AC "-", this will complete the relay coil circuit when the insulated section is occupied. The 12V DC source should be from a 12V DC wall-wart. These are inexpensive; most have a coaxial-barrel style connector but you can buy a screw-terminal adapter so you don't have to splice the power cable.