Let's start with some questions for you. Are you a runner (just like to watch trains run?) or do you want to be an operator (run somewhat like real world?)? Assuming you plan to make up a train from cars in your yard and back up a steam engine from the roundhouse to run the train, how will you pull that train out on to the mainline? Once you fix your yard connection to the mainline, how will you get your engine from the front of the train to the roundhouse after it pulls a train into the yard? Think arrival/departure track(s) with an engine escape at the other end of the yard.
Next, are you only planning to run one train at a time? Without passing sidings somewhere, that's really all you can do on an out-and-back layout. Your other choice might be to do a double track mainline with a double-ended yard attached to the mainline in two places. Have you tried the SCARM "run a train" feature? How did you like the results?
Is your overall available space only the 120" by about 280" or do you have more space at the ends of your length and width? If those are your nominal total room measurements, then your proposed layout will go wall to wall in many spots. How will you get to derailments in you yard or engine areas?
Where do you plan to put the operator controls and what will be the most prevalent view of an operational layout? If the "front" is near the bottom of your picture, then your up-and-over reverse loop will block the view of your turntable and roundhouse which are often times the most important (and costly) scenic elements of a layout. Why not put the up-and-over on the other end?
I strongly suggest you buy a track planning book (or see if you can borrow one from a local library) like "Track Planning for Realistic Operation." Even if you don't want to be an operator, it will explain ways to incorporate whatever features you desire into the space you have. I also strongly suggest you look at some of the other layout planning threads on this sub-forum and read the discussions about features included or skipped and the rational for each choice.
Read, learn, put into practice, and enjoy this hobby. You have a lot of space at you disposal (I envy you) to develop something that meets your needs (not just any old layout diagram), once you identify them.
Chuck
