To demonstrate the use of 1-3/8" isolation pieces (and to play with the new version of SCARM a bit) I took the liberty of re-creating your basic plan including these.
I also thought you may want to consider using O60 curves instead of O48 on the lower passing siding since the rest of the outer loop is O72. This substitution takes up essentially the same space and would allow you to run trains with a minimum O-54 curve spec on that siding.
The SCARM file and image are attached.