Plywood Thickness

It's the old thing of balancing between framing and decking: more framing allows the use of thinner decking. Also, will you be able to reach everything or does the framing and decking need to support your weight. I use glued 1X3+1X2 "L" Girder framing with 1X2 cross-joints and ¼", yes ¼" plywood decking. Joists about 10"-12" apart. And in a pinch (one corner I cannot reach) it does support me (carefully of course).

Lew 

My layout is hardly any larger than yours, and I needed to shave every fraction of an inch from the height so the grades between levels would be manageable. I am getting away with 3/8" plywood, with the upper levels supported by posts resting on the lower level. I try to have the posts no more than 11" away from each other in any direction. My track is Gargraves flex, and having that screwed down helps stiffen things up, too. It is hardly "best practices," but it is working -  I should mention that I am not running any enormous, scale die-cast metal steam engines on this, though some of the larger traditional locos are pretty heavy themselves. (I'm pretty heavy myself, too, but I haven't been brave enough to climb on it!)

I used simple "ladder-frame"  construction on my 15' X 18' layout, with 1" x 4" side rails and cross-members, every 16" on center, and 2" x 4" legs with a 15/32" plywood top and 1" foam and it is extremely sturdy and readily "climbable", even for an out-of-shape old geezer like me.

Like Lew said, if the framing is solid and tight, you don't need a really thick (3/4" +) top deck -  I think Mianne only uses 1/4".

With apologies to those who already know this, because it can prevent a lot of headaches for those who don't... Some people use the term "plywood" generically to refer to sheet goods.  All sheet goods are not the same. 

Real plywood is very strong - for a modestly framed layout, 3/8" will support a person who is walking carefully, so it will also support multiple levels of track.  In many applications, 1/4" is probably sufficient.  OSB (roof decking) is a reasonably strong alternative, cheaper, but it's heavier/thicker than 3/8" ply and not good if you're painting it.  Beware of particle board and particle-core plywood - heavy and not as strong.  MDF provides a smooth surface, but it's heavy, and the thinner sizes will flex easily.

And whatever you choose, be sure to support all the edges with your framing.

I ended up building my 16'X17' layout with 1X4 ladder frame construction with 20 inch centers and 2X4 legs. It's covered with 3/4" OSB. It's plenty strong and has been fine for 10 years. I've even gotten on top of the layout once or twice with no issues, but I'm only 5'9" and 156lbs, so take that as you may.

Go to a real lumber yard. 

NOT HD or Lowe's!

Buy two 5' X  5' X 1/2" Artic Birch multiply  sheets.  

Order a 5' X 10' Mianne benchwork frame.

Enjoy

My benchwork is built out of 1x4's with a 3/4" plywood top. Modules are 4'x4' with bracing @16" O.C. The support legs are all 2x4s. I'm a big guy and this supports me w/o issues.

My bench work hangs off the wall with no legs:

Grey painted 3/4" plywood knees cantilevered off wall.  Unpainted modules sit on the knees with cleats holding them in place. 

Always be thinking of safety: Note grey plastic conduit with white duplexes around the perimeter providing  20a 110v service for corded tools.  No extension cords ever on the floor to trip over.  Most hand tools used are now battery.  All long heavy duty commercial air hoses replaced with Slinky curley cords on mobile compressors wheeled to work site, plus a ceiling mounted air hose  All result of two nasty falls.

My layout is now 32 years old. I used 1x4's for the framing and legs employing L Girder construction. Where there are solid surfaces I used 1/2 inch A-C plywood which has been trouble free. 3/4 inch is more expensive, really heavy and in my opinion not necessary. The same goes for 2x4's. Just my opinions. Much of the layout is open grid as described in the the book "Model Railroad Benchwork" by Linn Wescott. (still available on Amazon) Open grid allows for grades and hidden storage yards and all kinds of features that a simple flat top table doesn't offer. Have fun. The layout building is the most fun in the hobby. One final note: Use drywall screws for all your benchwork construction. Final note: Gargraves track, Midwest cork roadbed and Ross custom switches powered by Tortoise switch machines. All built well, reliable and American made.

For fasteners consider #8 T-25 Star Drive Course Auger Thread type 17 point Construction Screws instead of drywall screws.   Available everywhere,  HD, Lowe's, Ace,  Grainger, etc

We stopped using drywall screws as soon as these became available many years ago.  Yes, one can get away  with drywall screws but one will kick themselves all day long when ever one need to remove a drywall screw where the head broke off.  Also. with a star drive you will not experience Phillips screw tips skipping around a worn out head.

A lighter (less weight) module.    Done with 3/16" luan board.  1" X 4" framing. 

Three modules.  "Y" module, Turntable and House.  These modules are a bit heavier. 

I really like Luan for decking. It has a relatively hard surface and holds screws well.

Lew

I use 1/2" Lauan from a REAL lumber yard. It is a special order. Look at the cut edges, the good stuff has distinct thin layers similar to marine plywood. But the "Lauan" you get at the big box stores has irregular layers and voids and gaps. 

We have a company here in Connecticut , https://www.connply.com/, that specializes in plywood. I don't know if they will sell direct to small buyers, but the lumberyards get stuff from them in a couple days.

Some of the Fort Pitt Highrailer finished modules. 

Confessions of a module builder - and I've built 20 with some of the oldest (25 yrs old) modules still in service.  Since modules have to be moved from storage to a venue, weight is always an issue, so a thin deck is the starting point.  1/4 Luan is readily available but I admit I have stock purchased years ago when the voids and de-lamination was minimal.   I would still stick with 1/4" to 3/8" for decks but will probably buy birch in the future to avoid quality issues.  Where you have to walk on the layout for access, 1/2" is adequate to support my bulk.  Deck flexing is solved with cross bracing (joists).  I use 1x3s for framing, 1x2s for cross bracing.  Boards are picked through at the big box stores to get straight pieces and stored to dry further.  Any twist that develops relegates the board to being ripped and used for short cross brace pieces.  2x2s are good for legs.   Conclusion: thin decks with good cross bracing works well for layouts.

For a fixed layout, I would not recommend decks thicker than 1/2".   I use 4x4 platforms for holiday train gardens.  Half inch plywood,  supported only at the edges of the 4 ft span, can be walked on.

Using 3/4" decks is overkill.  You are not building a house to code capable of supporting a 140 lbs/sq ft live load.  Your back will thank you for choosing a thinner deck.

+1 on using the Torx screws for frame assembly.  I still use drywall screws to attach the deck to frames.