Randall Styx says:

I have not used Plexiglas, and doubt I would, unless I wanted to create a “railroad in the sky” effect. While it could work, it is significantly more expensive than other options that will work as well or better. Those options range from solid wood to varieties of foam sheets.

The track and track bed can be supported by either a “right of way” method (where only narrow strips barely wider than the track bed are supported from a supporting frame) or a table-top method, where sheet goods form the base of most or all of the layout. These methods can be combined, with the table providing structural stability and elevated paths or foam risers supporting the track at various levels and slopes.

Which material is chosen to support the track depends on the priorities, preferences, and experience of the modeler. One who wants a permanent layout will appreciate heavier and more durable materials; one who wants a portable layout will like lightweight materials that are still stable enough to withstand moving; one who has a limited budget will be satisfied with less expensive materials.

Almost all the materials commonly used in model railroading today are wood based. Solid wood is an option and can work well for a right-of-way approach. If used to form a table, it will likely be built up of boards laid side by side. (It’s been a while since I’ve seen a single piece of wood one inch thick and thirty inches wide – and not only would that be very expensive, unless well sealed it would be liable to warp and split or check.)

One step away from solid wood is plywood. It is generally very stable. It is strong because the layers are glued up with the grain of each thin layer of wood oriented in different directions – commonly 90 degrees off the adjacent layers. It is also generally available and because the most common sheet size is 4′ x 8′ (in the USA) it is a good match for a table style layout for either HO or N “scale” layouts. (A trait it shares with other common sheet goods). High quality plywood, however, can be expensive, but CDX can work well when covered with scenery. The layers of plywood can be produced either by cutting multiple thin slices from a log or large piece of wood or by shaving a thin layer off a log as it is turned in a large lathe. (Thin slicing is more commonly used for making finish veneers.)

Next in line are sheet goods made up of “chopped up” wood. When the pieces are fairly large it is commonly called “oriented strand board” (OSB) or “chip board”. Because the wood grain is aligned in just about every direction, OSB is quite strong and stable; but because the pieces aren’t more than a few inches long, it is not quite as strong as plywood.

When the pieces are small it is commonly called “particle board”, or “press board”. At times these small pieces average about 1/8 inch long; at others they are more like sawdust. The smaller the pieces, the more the sheet relies on glue and the heavier it is, per given size and thickness. While all wood will swell with moisture, particle board is especially susceptible, for almost all the pores of the wood are exposed either on the surface or through capillary action through the material. While particle board is very stable and can be very smooth, it is not as strong as either OSB or plywood.

Next are sheet products where the wood fibers are separated not only by mechanical action but also by a process more like that of making paper, where the mechanically pulverized wood is cooked in a kind of soup. These are generally called fiberboard and can be pressed back together with glue and sealants to very loose densities (like Celotex and Homosote – both brand names, but the Homosote is generally a little denser than Celotex and will hold nails better). Low density boards are commonly used for muffling sound transfer between rooms or floors. They are very weak structurally. Fiberboard can also be pressed to a medium density (MDF) or high density (HDF). The higher the density, the stronger and heavier it is. MDF is widely available. Fiberboard can also be baked in the manufacturing process to make it especially hard – called “tempering” (like the brand name Tempered Masonite). Unless glued up with waterproof sealants, all fiberboard is subject to swelling on contact with water and water based paints and glues. New MDF might be very smooth, but when painted with latex paint can end up very rough unless it is sealed first.

While low density fiberboard is not very strong, it is much lighter than the other wood products and serves well to keep the noise level down, for it does not transfer the noise from the wheels on the rails like a sound board. It is also easier to cut. The lower the density, however, the more support it needs to keep it from sagging. (This is true also of particle board, which anyone who has had experience with particle board shelving knows.)

A final wood product that could be used, but seldom is, is corrugated cardboard, a true paper product that gets its strength from how the pieces of heavy paper are glued together with a corrugated layer between two flat sheets. Some can have multiple layers of corrugated paper. While it can be resistant to bending and folding in one direction, in the other it generally bends and folds too easily to be viable for a model railroad base, unless there are multiple corrugated layers oriented in different directions. But the other wood sheet products are easier to find.

The “new kids on the block” are foam products like styrofoam (white and visibly made up of small beads) and extruded polystyrene. While very flexible in thicknesses under an inch, in thicknesses of 1.5 inches and up are very stable and very lightweight. Styrofoam is very weak, and must be supported. 2 inch extruded polystyrene doesn’t need any more support than every few feet. Both foams are easy to work with, but neither holds screws or nails very well, if at all. Things need to be glued down, and one needs to be careful to use a glue that won’t destroy the foam. Wood glues can work to a degree, but the more common wood glues are designed to soak into the wood fibers a bit, and there are no fibers to soak into with foam. One advantage with foam is that, with practice, modelers can cut rocks that, with good painting techniques, look very realistic without needing heavy plaster coatings and rock molds. Even if opting for a hard plaster or Hydrocal shell, the foam provides a lightweight support system that is easy to work with.