Looks like a good start! I would think you'd stick with the double-mainline if you want to run multiple trains at a time, especially if young grandkids are involved. You could then still have an engine operating in the "green" yard & service area. I'm not sure how far you want the mainline elevated, but you can probably do a couple inches relatively easily, that would make it easier to see over the service area, especially for the grandkids. Would you have to access any of the "stay-out" zones with any regularity? Could be a nuisance.
I've noticed you're really close to having a reverse loop built into the service area of the layout. You may want to consider re-working the switches on either the top of left side to create this so that you can turn trains around if desired. You could also attach the far-right spur to the inner mainline as well.
I've heard that the curved switches can be a bear with smaller engines with a shorter electrical footprint due to electrical dead zones in the rail. I'm planning on needing a couple in the layout I'm designing, but something to consider, especially if smaller switcher engines are to be run.
Can't wait to see it develop! Good luck & have fun!
