I posted this track plan in the opening dialogue as I felt multiple references back to it might be necessary. (sorry for the lousy quality, I don't do computer drawings)
Basics: This is the bottom deck of my 2-deck layout, I call the maindeck. The big round circle at the top is my external helix 'room'. I also have a flat mainline dogbone loop in there (shown), and it has a turnout on it (at point X) that feeds the peninsula, then continues on around to become the other half of the 2 mainlines on that left side (at point B).
Those 2 mainlines continue on around the room, cross the entrance-way bridge, then up a stone viaduct to go into the helix structure to make their way to the top deck.
On the left side of the room there is a long straight track running right along the wall. That track feeds the lower staging areas by way of a lower helix track.
Along the right side of the room, there is another long section of straight track along the wall that acts as a siding that eventually loops around to feed the peninsula area.
I've not included some details of trackwork in several areas as I did not feel they were necessary to this discussion of reversing loops.