I'm not an expert on yard design. But if possible, I would make the yard lead long enough to double a cut from one track to another. The way it's drawn right now I think the lead is a bit short. Personal preference, but I like the end of the lead to blend into the main.
Another thought... I see you are using a Ross 4-way crossover near the upper left side. I'm not sure how prototypical these are. On rural double-track, I seem to see crossovers made up of individual pairs of switches placed sequentially. I recommend three or four crossovers between the mains in each direction. Unless you want to be locked into right-hand running with one train per mainline, adding crossovers together with today's command control will give you a lot of operational flexibility. For example a freight can work a siding on the opposite main, and you can cross a passenger or through train over to keep things rolling. "Any train on any track at any time," at least CTC if not PTC! 
Last point... You mentioned a Challenger. It sounds like you're a UP fan and it's inevitable that you'll eventually get a Big Boy. Maybe a Veranda turbine too. These locos have a LOT of overhang, so make sure you have adequate clearance between main lines. The articulateds, especially, will look better if your curves are EASED. If you don't want to sweat the spiral geometry with flex track, just lead into your O72 sectional curves with a couple of wider ones. For example, make the first curve 096, then O84, then O72 or whatever the geometry requires.
Another observation: You have a lot of standing room in the middle. Don't be afraid to leave a little platform area inboard of the track. It might prevent your trains from falling to the floor in a careless moment. It will also let you add a little scenery between the track and the inside edge. But don't overdo it-- folks will have to reach around any scenery there to operate and uncouple. Last point (yes, I have a lot to say for a guy who hasn't put hammer to nail since 2005!) Try not to have your track run rigidly parallel to your benchwork. Add a "scenic kink" for visual interest!
It's easy to sit on the sidelines and say, "do this, do that." You're so blessed in that you have sufficient room to to it "RIGHT," and it's exciting to see it all come together for someone!! Thank you for sharing your plan and enduring our criticism. If you make no further changes I'm sure it'll be GREAT!
