Mark and J: I will start with a description...I have included some photos, not sure how much they will help
The basic idea for the larger modules starts with using the L - Girder Benchwork method. 1 x 4, plus 1 x 2 (or 1 x 2.5 inch tops forming the L).... The joists are permanently attached to the table top, in most areas 1/2 inch higher quality plywood, then across the bottom of the joists I ran (2) 1x2's to act as stiffeners. All the wiring for the module runs through holes drilled in the joists and terminates at each end in terminal blocks, or cinch plugs or a combination of the two..... When I took them apart, I removed screws that I marked "remove" between the joists and L girders and I could slide the modules apart without having to do any major heavy lifting... Same goes for re-assembly... There are a number of reprints of the book done on this L-Girder, method of construction, worth every dime I paid, and I bought one used at a train show for $3.00
If your familiar with something called the cookie cutter method, I was able to cut the plywood and remove sections to allow for depressions in the scenery, and also to facilitate smooth grades for the sub-roadbed.
After the move to my dad's home I upgraded some of the L-girder joists to 1 x 4 mahogany decking boards, it's much stronger than pine, if you pick pieces with really straight grain, it is very stable, meaning it wont warp or twist, and I can have L girders that approach 14 to 16 feet in length with only two sets of legs. I hate having too many legs, as I inevitably spend some time under there on the creeper I built and the legs are a pain the butt to move around. All the legs, have 3/8 x 16 carriage bolts threaded into "T-Nuts" for height adjustment. All my legs are 2 x 2... In my case I had kept the original 2 x 4 's from my first layouts in NJ, about 45 year old lumber.... I ripped the 2 x 4's into 2 x 2's because they were incredibly strong, straight lumber... Back in the good old days before genetically altered lumber (pulpwood)....
On the major 2 level area where the engine service facility meets up with the Main lines, I lowered the L-Girder frame work about 9 inches, and laid the plywood I needed for sub-roadbed on the lower level, then carried the supports off the joists to support the upper level. As it wasn't practical to have joists for the upper level, I added 1 x 2 bracing to the upper 1/2 inch plywood running parallel to the trackwork, ie parallel the L girders... but again, by making the L-girders separable from the joists and table top things can be moved around and positioned.
The most recent 2 modules the coal mine and coal run are slightly different... 2 ft. x 8 ft. frames purposely made to weigh less than 45 lbs each so I could move them.... 1/4 inch plywood with 1 inch thick pink styrene plus another layer of 1/4 inch plywood under the track at the mine.... the mine run is 1/2 inch plywood sub road bed with 3/4 inch oak and pine supports under it for stiffeners so that I could raise and lower the entire roadbed and keep it straight. The modules slide in an out of place on some 3/4 inch wood support rails and then screw together.
Major modules with numerous switches have their own control panels, that way switch wires and controls don't have to leave the module, signal relays were assembled on a bench and mounted near each block signal, the leads to the signals themselves go through small 4 pin connectors so they are easily removed by unplugging them.
The large shallow relief mountains were built off the bench and put in place, except for the Coal mine and Mine run modules where they are part of the module. For these I screwed 1/4 inch plywood to the back rail of the module then when I added the spray foam and mountains etc, it became structurally part of the module. The large mountain to the left of the Coal Mine was made in a module off the bench on a 1/4 inch plywood base then moved and sits on the benchwork with some struts coming down and screwed to the frame below grade.