Gatorfoam® is the material from which I build my layout's modules framework. Gatorfoam is a registered trademark of 3A Composites (the company's name has changed several times over the years). They are based in Davidson, North Carolina. The trademark was applied for in 1976, so this material has been around for a number of decades.
(external link: Trademark Info)
This photo shows my first order of this material. In 2021 ten sheets of 24" x 36" x 3/16" cost about $150. I would have loved to have used 1/2"-thick material, but the cost goes up dramatically as the material gets thicker. And, I actually need more sheets rather than thicker sheets, based on the honeycomb design I am using for constructing these module bases.
Gatorfoam has a foam inner core that is similar to the common foamboard you can buy at hobby and art supply stores. What makes Gatorfoam different, is that its two outer layers are made of a wood veneer (similar in concept to plywood), but that wood veneer has been impregnated with resin. This makes the board's surfaces very resilient, and it virtually eliminates warping. These are two common ailments of nearly all foam-based sheet goods. Various PDF files are available from the company's web site.
(external link: 3A Composites)
This photo shows the ten sheets that I ordered. As you can see, they are perfectly flat. The surface is very hard, and you will need woodworking tools to work it. Regular foamboard can be cut with a utility knife. This material requires some sort of powered saw (e.g. a table saw) to cut to size, and a drill to drill holes into it. The company states that it can be cut with a utility knife, but my attempt to do so was unsuccessful.
Gatorfoam is available in 3/16", 1/2", 3/4", and 1" thicknesses. The surface color can be white, black, or natural (my preference). Additionally, the foam color can be either white or black. Wholesale Gatorfoam is available in 4' x 8', 50" x 100", and 5' x 10' sheets. However, there are resellers that sell them in more manageable sizes, which yields more affordable shipping costs as well, for us individuals. Artgraphix is where I bought my ten-pack. The sheets were extremely well packed, and they arrived intact.
(external link: Artgraphix)
One thing to note in working with this material, is that the veneer layers are ever so slightly bigger than the foam layer, in both directions. So, if you are trying to glue two pieces perpendicular to each other, they won't touch over the entire width of the thickness. I used my table saw to trim just a sliver off of the mating surface. Also, be very careful of running your fingers or hands along the edges of the board. The veneer layer is very hard and somewhat sharp, so you will wind up with something similar to paper cuts. I use gloves when I am working with this material.
This signmaker used Gatorfoam professionally, so he used it to construct his layout.
(external link: 3A Composites Features Model Railroader)
An example of an N-scale layout whose sub-roadbed is made out of Gatorfoam:
(external link: Building a N-scale Layout)
The owner of this layout shows how he installed sheets of Gatorfoam for the backdrops of his layout.
(external link: Backdrops from Gatorfoam)
This MRH blog posting shows how an N-scale modeler built a 21" x 8' extension to his layout using Gatorfoam. The material is strong enough to mount toggle switches and Tortoise switch machines to.
(external link: 8-foot Long N-scale Section)
A page showing various layouts and dioramas that have been built using Gatorfoam. Gatorfoam.net is a web site run by the same guy that is featured in the 3A Composites linked-to page above. He sells 4'x8' sheets of Gatorfoam, but only for local-to-him pick-up.
(external link: Gatorfoam Modelers Gallery)
Just some confirmation that Gatorfoam doesn't sag or warp.
(external link: Testimony of Using Gatorfoam)
Shows an introduction video to a paid video about how to use Gatorfoam to build large structures.
(external link: Building Structures out of Gatorfoam)
The owner describes building a module out of 1/2"-thick Gatorboard.
(external link: Tam Valley Railroad Roadbed)
A couple of examples of building mini scenes on Gatorfoam boards.
(external link: Gatorfoam for Display Models)
Art Fahie talks about Gatorfoam.
(external link: Bar Mills - About Gatorfoam (video))
The layout owner shows his N-scale layout's valances made out of Gatorfoam.
(external link: Gatorfoam Soffits (video))
Fellow S-scale modeler, Roger Nulton, uses Gatorfoam as the backbone of his larger structures that he scratchbuilds as the material doesn't warp.