@Madockawando the pictures you posted look as if it may be possible to insert 1-3/4" pieces (shown in blue) into the short ends next to the 10" straights of the outer loop as @Mike0289 suggested and as shown in the most recent plan I posted above. If this were possible, that plan may work to accomplish your stated goal.
Here is another possibility with a center to center distance of 4-3/4" and inside roadbed distance of ~1-1/4", using two 4.5"s and one 1.75" straights on the ends.