Here is a good example of a "module" of the open grid construction. If you can sit on the assembly without it collapsing, it should support all the plywood, roadbed, plaster patch, trains, scenery, etc. Just don't sit on it when the joint compound is wet!
Our previous railroad was all on a flat surface with a few minor elevations. In our travels we encountered some large scale layouts, most of which were using some variation of an open grid system. We thought if they could do it, so could we. It's more difficult than building a flat railroad, but much more interesting and challenging. The results are far more realistic.