As have been previously stated above, combining several of the posts, you need three things to make the DZ2500 derail feature work.
First - common ground between the track power source and the DZ2500 power source. You have written that you have already accomplished this.
Second - the isolated rail sections. I am pretty confident that with the Ross Switch, and the fibre pins you installed between the tubular track rail and the switch track rail you show in your picture, you should have the isolated rail sections handled. You can verify if they are truly isolated, but I will write about how a bit later.
Third - connecting the green and yellow wires from the DZ2500 switch machine to the isolated sections of rail. Green of the DZ2500 to the turn out isolated rail, Yellow of the DZ2500 to the straight thru isolated rail.
Like activation for some accessories using the isolated rail technique, the concept is that the wheels/axle complete the ground connection from the non-isolated rail to the isolated rail section.
To determine if you have isolated the rail sections, I would disconnect the DZ2500 green and yellow wires from your isolated rail sections, but you can leave the wires you have already connected to the rail sections in place, just let them hang in the air. The best method to me is to use a Volt Ohm Meter to measure ohms. For this method, the power should be off to the track and DZ2500. With the meter set to measure ohms, just keep the two leads separated in the air, and see what your particular meter shows for an open circuit. Then touch the ends of the meter together, and see what you meter show for a short circuit (should read 0). Touch one meter lead to the isolated rail section, and the other lead to an outside rail you know is not isolated. If you have the isolated rail created successfully, the meter will show the reading for an open circuit. If not, the meter will most likely show 0, or close to it.
If you don't have a meter, an idea I have is to use an accessory that has a light bulb in it. Again, have the DZ2500 disconnected from the isolated rail sections. Apply track power. Touch one lead from the accessory to the center rail, and the other lead from the accessory to a non-isolated rail, and the light should come on. Now, keeping one lead on the center rail, touch the other lead to the isolated rail section, and the light should not come on, thus indicating ground is not present, the rail isolation actually exists.
Once you have convinced yourself the isolated rail sections exist, connect the DZ2500 switch machine Green and Yellow wires correctly for thru and out to your wires previously soldered to the isolated rail sections. Turn on all the power, have a car on the track you can move with you hand outside of an isolated rail section, use the button on top of the DZ2500 to put the switch in the opposite position from where the car is, then push the car into the isolated rail area, and the DZ2500 should change to the correct state.