The Atlas #200 is a Double-Pole, Double-Throw latching electrical switch. It uses a twin-coil solenoid to shift the contact from one side to the other. The solenoids are actuated by MOMENTARY pulses of electricity, just like the Atlas turnout switches, except that the #200 instead of actuating the turnout points, actuates the connection to two separate contact circuits, to which accessories can be connected. If you are using Atlas switch machines to throw the turnout points, or any other twin-coil switch machine (Walthers, Kemtron, etc), you can wire the #200 to the same source as the turnout machines. You CANNOT connect the #200 to stall-type switch machines, unless they have separate contacts for accessories. (But then you wouldn't need the #200.)
As to using a 3-lamp signal, I take it you have red, yellow and green lamps. Let's say you want only the green lamp lit when the turnout is for the straight route, and yellow-over-red (assuming red is on the bottom, yellow in the middle, green on top) is when the turnout is aligned for the diverging route. (Yellow-over-red might mean slow speed, not to exceed 15 mph or somesuch). You would connect the red and green lamps to one pole on the relay, with green connected to one side of the pole and red to the other side. You would connect the yellow lamp to the other pole, on the same side as the red lamp, but NOT on the same pole. There would be nothing connected to the other side of the pole to which the yellow is attached that corresponds to the connection on the other pole that the green lamp is connected. Keep in mind that there is NO connection between the actuating power of the relay and the DPDT contacts, so you can use lower voltage or even batteries on the LED's. Just be sure the LED's have resistors in series with their power supply.
I'm not sure how to post an actual diagram, but I'm sure someone else will.
