[Edit] Excuse the formatting, seems the board wasn’t playing nice for me when I had set this out any help here much appreciated thanks 
(the Rich Text or HTML editor I could not find to reformat this) .[/Edit] I have been reading through the Layouts and Layout Building section of the forum and something struck me. Am I the only one who uses entirely foam rather then plywood for the base of their layout? I use a mix of scrap foam plus the Woodland Scenics Mod-U-Rail System to build the base of my layout rather then plywood after which once the module is near completion I will build the framework for the module to sit on, followed by joining the module to a pre existing one allowing slow but gradual expansion of my layout to its full planed size. So in a nut shell; opinions, thoughts, (constructive) criticism of me using foam over plywood and also what do you use and why? I am interested to gauge the community’s opinion as so if required I can switch over before the layout is too far along in construction. Some basic info on my layout: HO Scale, Set post 1990, Freelance Coal to Synthetic Oil Refinery with adaptations such as cars driving on the left hand side of the road (I am a New Zealander), Modules complete are the Engine Terminal at the refinery end of the layout, Modules under current construction is the refinery itself and associated yard, DCC planned to be used. More info on my layout being built can be found at http://bens-hotrainset.blogspot.com
I asked a similar question after a few heated discussions. I got curious myself.[:)]
http://cs.trains.com/trccs/forums/p/169350/1859709.aspx#1859709
Brent
I use 2-inch foam over a box frame. I have no plywood underneath. I don’t need to climb on the layout, and I see no other reason to use plywood.
Recently, I’ve installed my first Tortoise machines. Here, plywood is an advantage, as you can mount them directly to the bottom of the layout. But, I used 1/4 inch plywood, cut into 3-inch squares, and glued them to the underside of the foam. So far, no problems.
My subgrade is cookie-cut plywood, supporting roadbed cut from thin foam (fan-fold underlayment.) Everything below that is steel - steel studs, erected in classic Linn Westcott fashion: ‘C act like L’ girders, joists and risers. The primary purpose of the plywood is to accept screws.
At present, most of my layout surface on any track level is - air. It rather resembles a Disney thrill ride under construction.
Exactly how that will be supplanted by scenery (on the levels meant to be visible) is still under study.
Chuck (Modeling Central Japan in September, 1964 - when I get a round tuit)
Because I built mine to be somewhat move-able, I used a box frame of 1x3’s with a sheet of Luan on top, and a sheet of one inch blue foam on top of that. It is very light weight and sturdy.
Otherwise I would have gone with L-girder like I have done before.
I’ve used both Homasote/Celotex on plywood and foam on plywood with WS risers for my main BLT N-scale RR layout. I’m not so fond of the WS foam method. The plywood is mounted atop kitchen cabinets like a countertop.
I took an idea from boatbuilding and aircraft construction for something totally different for the small Cha-Cha Chesapeake Industrial RR (also N-scale) and it appears to be working well: a sandwich of foam/Celotex. For this layout, I wanted portability and light weight, since it rests on a couple of Ethan Allen base units but may go to another room or another house sometime soon. The photos below show the idea.
First, I cut the 2" foam and the Celotex to the same size for each of the two sections of the layout:
Next, I removed the plastic cover from the foam and spread it with Liquid Nails for Projects, aligning it again. I weighted it with a bunch of handy heavy items to ensure a good bond:
The result is two rigid structures (19"x30" and 19"x40") that hold track well, are easy to drill through for wiring, quiet the trains and weigh only about three pounds.
I’ll probably put a low box framework (perhaps 1x2) beneath it and an edge of Masonite for a fascia to mount switches and hold the alignment of the sections, but that’s about it. I plan to keep the framework and the sandwich separate, so they can be easily lifted and moved.
It’s a work in progress and I’ll know more about its success in a few weeks.
All of my layouts have been plywood. Recently I set up a test track oval on foam over a table and a desktop. There didn’t seem to me to be any real advantage to it over plywood unless weight is a factor. I can see using it for scenery, however, and will probably do so on my next layout.
Enjoy
Paul
Mine is 2x4 framing with 1/2 inch plywood and 2 inch of foam on top. It doubles as an air raid shelter. [:)]
My layout is built on 1x4s with 1x3s for my L-girders. I use 1x4 supports and for subroadbed I use MDF. I have my supports
I glued Midwest HO cork to the MDF with Elmers Carpenter glue. All track is Walthers code 83 spiked to the layout with either Walthers spikes or ME short ones.
Smooth and reliable after all this time…and I live in humid Houston!
Roger Huber
Same but I use ground throws on little bits of ply matching the cork heights.
Hear is some pics of me using entirely foam for the base of the layout, before the modules are put onto their purpose made wooden stands 
Some shots of using foam entirely Interesting feedback on this thread and very good to see Hope to see some more as other beginners (like myself) might trundle along here and weigh up their options on what’s is posted here 
My blog (see my profile) has more on my layout
Most benchwork for my Santa Fe in Oklahoma layout was started in 1984, and is 3/4inch plywood with homabed roadbed glued down over it. It is pretty quiet, easy to maintain and I have never had any reason to change. In fact, all 3 decks of the layout, which is 33ft by 29ft is built the same way and is solid and sturdy today. I have nothing against foam, to my knowledge I don’t know any modelers who do use it, but I will stay with my “tried and true” to the end.
Bob
Most of my layouts have been built with plywood, either full tabletops or cookie-cutter style. My current layout was built entirely of foam, mostly 2". It was originally built to sit on an old kitchen table but when I added another section, I built an L-girder frame to fit under the foam layers. The L-girder sat on the kitchen table, too.
Later on, I decided I wanted the layout higher up so I built rectangular frames to place under the L-girders, tying the frames together with some angled braces. It all still sits on that old kitchen table! LOL
I plan to build a larger layout sometime, probably around the walls. I’m still thinking about the type of construction I’ll use for it. I like the idea of shelf brackets but I want multiple levels and the brackets interfere with nolix-style construction (no helices for me!). It will probably end up being a combination of 1x2" lumber with foam topping.
Darrell, quiet…for now
1 X 4 X 3/4 wood frame on 16 inch centers - glued and screwed.
3/8 plywood top - glued and screwed.
1 inch foam glued down with a few spots of latex calk.
Cork roadbed very lightly fastened with latex caulk.
Track very lightly fastened with latex caulk and a few track nails.
HO scale
I have some expansion in summer. I allow for this by leaving rail gaps every 8-10 feet or so.
I remain a proponent of 1/2" plywood.