I like your idea

I think you have a good plan.  I would make sure the wall is attached well at the bottom and the top. Can you tie the wall into the round support?  I would also use metal brackets between the wall and the cantilevered supports to make sure they never come loose.  Or/and you could capture the cantilevered piece by doubling the 2X4 stud.  You may want to consider making a 2X6 wall, but I may be entering into the realm of overkill.  

"You may want to consider

  A 2 by 6 or even 2 by 8 wall would give better leverage for the deck cross support connections. A 2 by 4 wall might need metal angles to keep the deck supports level but a wider wall could stabilize the the cross members without need of brackets. Two 2 by 4 walls spaced a bit apart would be even better. The weight of the layout is not the load to worry about, the weight of builders leaning on them in awkward positions is what needs to be considered.....DaveB  

Take a look at the "B&O/PC

Take a look at the "B&O/PC Cincinnati West - Construction" thread on the first page of this very sub-forum. If you are going to have 4 layers, you need to keep the support structure thin, and something like what R. Seiler did with 1x1" square tubing would fit the bill - and result in a thinner supporting wall as well. You'll want to go floor-to-ceiling with the verticals and attach them somehow up above. 

Advantages:

thinner vertical wall and layer support

stronger than wood, reduced sag potentential

Disadvantages:

- you need to source 6' or longer 1x1 square steel tubing

- you need to weld it together or find someone who can

- probably more costly than wood\

- you need to figure out how to fasten your plywood to it

If you use perforated square steel tubing you might be able to bolt it together, like an erector set, and avoid welding. 

Steel studs

Hi Espeelark,

Steel studs would allow you to support the upper decks and would not "droop" over time.

Keeping each deck as light as possible would also be a good idea.  LED lighting

You could also conceal a few threaded rods for additional support.  Or go high tech and use 1/8 rods or cables.

This would require tall trees, smoke stacks, double sided backdrops or mountains to conceal them.

All great ideas!

All very good ideas guys!

I like the idea of a wider stud wall using 2x6 or even 2x8. I'm thinking that the slightly canted stud wall on the left side of my sketch could be two, parallel 2x4 stud walls which would giver me a wider "stance" to support the cantilever loads as well as more easily allow horizontal supports, fastened to the "inside" of the two stud walls to extend to the left to support the left edge of the blob.

Also, to address "Akarmani's" question, yes, I can extend the stud wall up and tie into the floor joists above.

"Eastwind" - I happen to know a guy if I decide to go tubular steel. I'm part of Randy's crew building his B&O/PC Cincinnati West double deck layout, so I'm sure some kind of "agreement" could be achieved.

Thanks again! Any more ideas?

1" metal...

maybe extrusion also known as 80/20 would work for this? may be a little expensive though

Steel...

I agree 1" square steel would be the way to go like that guy building the B&O/PC Cincinnati West. 

He's a freakin' genius I tell ya. 

Randy

How wide is the benchwork?

  at the widest spot?  The narrower the benchwork the less the strain on the central connection. 16 inches or so needs nothing special 30 inches probably needs metal shelf brackets to avoid bending down when leaned on. Steel framing would be fine but it's not necessary and costs a lot more plus has other dis-advantages. 2by4's are cheap and easy to assemble with common household tools. I wouldn't use steel unless you already have the tools and the skills to work with it ,and if you did you'd probably not be asking these questions :> ) I used to build race cars so could easily frame up a layout in steel but I still use wood for my own benchwork.......DaveB

The 2x4 vertical wall will be

The 2x4 vertical wall will be fine. They are quite capable of being a load bearing wall on a house so they will easily carry the load of a model railroad. Easiest way to go thin with supports is to run them as one piece though the wall and glue and screw them to the studs at the appropriate height. I would use 3 inch strips cut from 3/4 inch plywood and double them at each stud. You would then be able to stand on them with no sagging and be less likely to warp and have a good surface to attach each level to. You could even us 3/4 inch material for your table tops and still be able to put a bunch of full grown model railroaders on there with no sagging.

2x3s can handle the load... more to consider

The 2x4 is actually overkill, but modestly so.   2x3s could do this.  With sheathing or drywall 'skin' added, this is very sturdy, but not necessarily "balanced" to stay vertical.

If your able to fasten to the ceiling joist,  it's all good, and I take that back.   My suggestion would make a floor to ceiling joist wall / partition.

Were this mine, I'd try to spread the peninsula end  (to a T shape... or branching into a Y....  or a cylinder at the end)  just to make the equivalent to a tripod.   Thus, you could cantilever out to both sides more easily than relying on a single panel to stay in place as (train) weight shift from one shelf to another.

Survey says, "wood"....

Great feedback / comments!

Dave B - the sketch at the top of this thread contains a grid that are 12" square. So, the widest part in the blob is ~36".

Peter - I haven't quite decided yet, but will do something on the left-most stud wall. It will either be two, offset, parallel 2x4 stud walls, or a "T" as you suggest.

I commandeered a few minutes at Randy's B&O/PC CIncinnati West Friday-night build session to discuss the options presented here. I believe the concensus was that if I were to use plywood L-girders or T-girders cantilevered off of the stud wall, that would be more than adequate. Granted Randy's 1" square steel tubular framework (which was staring me in the face) provides a lot of strength in a small package, wood suits my skillset a bit better.

Question: Knowing I'm going to use plywood "L" or "T" girders, what's the thinnest plywood I can construct them of and and still have it be strong to support a small elephant? 1/2"? 5/8"? 3/4"?

" the widest part in the blob

   That's wide enough to get a sturdy central wall. I'd do it tear drop shape following the curve around the left end  and as wide as possible on the right end. If the shelves on opposite sides of the wall are the same height I'd run the cross members full width across .If the elevations are different from side to side you'll have to design a suitable connection for shelves to wall. Perhaps metal shelf or construction framing brackets. Anyway it looks do-able from a shelf construction standpoint. I'm not fond of the density of the design from an aesthetic or maintenance  viewpoint however....DaveB