You don't mention how high the layout is. You could do a huge staging yard directly under the existing layout using three sides of the same footprint as the main layout. I would make the staging yard a big letter "J" with the long end of the J under the long side ending where you have "city" marked on your drawing. Then the "J:" continues along the bottom side in the drawing and up to end the short side under "industry." That leaves the diagonal side marked "ATSF" with the passing siding for your duck under. If I'm figuring correctly from the measurements you gave, the distance from the bottom left corner at the end of the Industry bench around the corner up to the city is 9 feet by 11 feet or 240 inches. A six inch rise over that distance would be a 2.5% grade. I think that would allow the staging yard to be placed 6 inches below the main layout height. If you want to put more separation between the levels and still keep a duckunder but not have too extreme a grade to climb, you could use two switch backs with say a 6 foot tail track on each end. The train length would be limited to 6 feet, but I don't think you would want to have a train much longer than that on the mainline since it would get too close to chasing it's tail if it was much longer than 6 feet. With some automatic reversing switches and automatic turnout controls, you could start a train up the switch back and then let the switch back tracks work the train up automatically. It should join the mainline somewhere in the "city" allowing the area under "ATSF" for a duck under. Make your staging tracks far enough apart that you will have plenty of finger room to rerail trains.