Flip

May seem like a simplistic answer, but can you flip the layout end for end on the shelf, making the front the back and the back the front?  That puts the main track towards the front and might make it easier to run a return loop behind the layout.  You could even relay the last 8" of main track to curve them more towards the front (current back) which would reduce the depth.

Curving the last 8" or so towards the front (current back) would also let you adjust the turnback loops, effectively making the layout "shorter".

The challenge with flipping

The challenge with flipping the layout is that I have 4 industries as low relief structures on that rear track.  There isn't really room to model them if they are on the front edge.  I might be able to expand the room between the industry track and the edge though to get a bit more space for the structures.

low relief structures

You can always model the low relief structures on the front and detail the interior.

Leave the doors open and viewers can rail fan through the buildings.

Why not flip,

and have the back section go behind a backdrop? Then the train enters through a tunnel , from under a bridge, or from behind buildings, runs out front, then disappears the same on the other end. You get continuous running and isl at the same time.

Simplify

 Why not just put turn back loops at each end and run back across the scene on the same mainline. Auto reversing would handle the loop polarity and a spring switch or detector circuit could throw the turnouts. If the end loops were big enough you could add sidings on the loops for staging or passing trains....DaveB

Lower Level

As another alternative option, since you already plan on a helix on one side, why not have helixes on both sides, and have the return run below the scenic level.  That would make access simple, and also easily allow for a couple of staging tracks.  I realize adding the two helixes would somewhat complicate construction.

Scenery blocks

While everyone else has the trackage covered, I will add to the scenery block portion. Model trees that are in scale and you will find you can hide the loop pretty easily. Some mature oaks, maples, and others will be much bigger than you are likely used to seeing on models and the canopy can cover the track.

Dan

Hide the train you're watching?

If the goal is to watch trains run, making well over half the run hidden trackage by either helixing down to a hidden lower level or running it behind a backdrop or trees would seem to defeat the purpose of the change.

I like Dave B's suggestion to stick a return loop at both ends, if they'll fit.

What does ISL mean?

You migth want to make a city scene as a view block.

Tall building in and around the return loops would also provide a view block, probably better than a dense forest.

I think "ISL" means industrial switching layout.

Helix question

I have a question on the grade in the helix.

7.5" radius is 15" diameter, which means a 47.1" lap.  If you rise 2" in 47.1 inches that's a 4.2% grade.  If you rise 3 inches per lap, that's 6.4% grade.

In order to get a 2.5% grade in the helix, you would have to have a separation between levels of 1.2".  Its been a while since I was in N scale, but that's pretty tight. Or increase the radius in the curves to something in excess of 12" radius for 2" rise or 19" for a 3" rise.

How comfortable are you with a really steep grade on a really long minimum radius curve?

ISL

What does ISL mean?

Industry Switching Layout or Industry Switching Line.

Alternative

Turn back loops

     I'd widen the benchwork at the ends  and use a 11 inch radius main line loop at each end with a 10 inch(minus a bit) passing siding inside the main. This would give more trouble free operations for longer cars with body mount couplers  if ever desired. A passing siding at each end would let you run multiple staged  trains in either direction or could provide a minimal yard at each end of the run if desired....DaveB

That loop on the left is very common in the UK

I forget what they call them (T?L maybe), but you leave town and take your train into the loop and close the switches. Then let your train remain going around in a circle for a period of time. The loop is hidden. Then once a sufficient time has elapsed, you have your train exit the loop and enter the next town. It's a way to eat time so that your train isn't suddenly in town B after leading town A or your head end in town A and the back end in town B.

Obviously it works better with shorter trains. 

Lots of great ideas here

Thank you all for the great ideas. I realized that I forgot to mention that the main viewing angle is from the bottom. I have drawn up a fourth option that includes your feedback.

This idea utilizes the full 12' of length that I have available for the left hand turn back and the three original modules. 

The right hand turn back would be a drop leaf.   The turn back radiuses are 10.5 on the left and 8.5 on the right.  The main reason for the smaller radius on the right hand side is so that when the leaf is dropped it doesn't stick out past the bookshelf.

In addition I would need to flip (re-make) the left hand main module so that the trestle curve naturally sends the track to the front of the layout.  I would end up sliding the main modules forward on the bookshelf an inch or two to allow for the return track in the back and would end up with a view block or backdrop between the main modules and the return track.

I could add a staging track along the back of this design.

Thoughts?

v2

It seems to meet most of your criteria, ISL, continuous run, fits space and it has none of the challenges of the first plan, grades, helix, minimum radius.

V1

Hi,

I like the track plan in V1 - with the return low at the front of the layout. Obviously the under-the-river part needs to be in a tunnel (which might even be open to the front). The rest of the return run can be modelled as a "mainline in an open trench" and be used to "parade" the train. It can also serve as a fiddle yard. But basically you can use the lowered front return run as part of your story. I wish I'd thought of that when designing my layout!

Anyway, have fun!

v2 looks good for roundy-round

No one ever regrets using a larger curve radius!

But as Prof K says, if automated back-and-forth is acceptable, you can do that with NO track changes.