Richie
I'm pretty sure Carl's comment about 11 3/8" was referring to the gaps in my drawing. The Lionel catalog says the 031 and 036 switches are both 10" long.
I tried a version using 036 switches on the top and bottom only, and them changing to 031 after that. It is tight. I could not get it to fit without the first switch in the top right corner being halfway into the tunnel portal.
So to me it seems to be either all 031 or fewer sidings. Personally, I'd go with all 031.