Feedback on layout plan

Here is a 3d view of the proposed plan with the tunnels at the two turns ....

You have reversing (S) curves coming out of the tunnels in both loops. I don't know if there is room to relocate the turnouts closer to the tunnels using the curve of the turnout to continue the loop and then run the straight part up to the main or if it would be better to move the turnouts further from the tunnels with a straight piece and then move the mains away from the edge of the layout. Given the layout dimensions, I'm assuming those are O36 curves/turnouts and I think the S-curves will give you trouble unless you run short trains and slow down.

Yes these are O-36 curves. Wanted to use 0-48 for those sections but given its a 30 inch straights on the two arms of the L I can't swing larger even if the ends will accommodate them at 4 feet. 

May I suggest adding a longer passing siding.  Let me explain.

Assume your are traveling in a counter clockwise direction and coming out of the lower left tunnel.  You take the divering reversing loop switch and place another switch which diverges to the right then the passing siding follows the main line around to the upper right reversing switch and then you have a nice long siding for passing.

The passing siding in the top portion of the layout is probably just too small!

Looks nice.  From my experience,  Sidings and dead end yard lines can be tricky.  My yard lines are 7 feet and it is not enough, I wish I had 10 feet.  If you are going to put a Passenger train in the passing siding then figure out how long that passenger train - nose to tail is.  you want to make sure it fits when you start running your trains.

I know space is a premium, but if the sidings are too short you wont use them.

You absolutely positively want to include at a minimum one more passing siding.  If two trains are operating in different directions one train would be continuously "stuck" at the passing siding.