Okay, Mark, here's the first version, sorry it took so long. 
Please note that in order to fit a siding in, I had to restructure the lower loop so I could move it over. I also noticed the right side was crowding the edge, so I moved that in too. The result meant shortening the yard lead as well as the original passing siding. As you can see I added another spur in the yard for some extra storage. Whether or not you add it to your build is up to you.
Based in where you drew it, I had to put the upper switch for the new passing siding on the same level as the upper loop (8") because it's so close and the experts say not to start/end a grade at a switch. The lower switch has a 6" elevation. The grade down to it is 2.2% and the grade from it to the bottom is 2.7%. Changing the elevation to 5" changes the grades to 3.5% & 2.3% respectively (5.5" to 2.9% & 2.5%).
I guess the bottom line is that you can add a siding and I'm not sure what else needs to be done. We can fine tune things, but I think that's something you should do during construction, assuming this basic design works for you. Me? I pretty much like it. I think it's got a good balance of track to area and leaves room in the upper loop and the right side for buildings, etc. If you think the yard is too big, just don't add the 3rd spur.
So, what do you think?
