Reply to "Help me design an attic layout"

Congratulations on getting your benchwork roughed out.  As Frank (Moonman) suggested ease those sharp hurtful exterior corners before much railroad work is done. Also avoid a duckunder ( if considered) by installing a piano hinged drop section which you can disguise as a plate girder bridge. Jim Barrett did a video[#11) of "how to" as well as parts,etc. You will seriously regret a duckunder as others have said, yet it will provide much flexibility in track planning for longer runs. My drop section pictured below  was built from Jim's long ago article in O-Gauge Magazine and is rescued from two of my former layouts.