I must have done something wrong when I built my last layout. It was built using 1/4 luan and 1x2s spaced every 12" topped with Homasote with everything glues and screwed using Titebond and drywall screws. With support around the edges and a couple girders running down the center it could more than support my weight while I walked/worked on it and it had a 60" width . I did it this way to make it easier to disassemble and transport it when it came time to move and it worked out great. I was able to build and move a 5 1/2 x 10 1/2 foot 3 level layout by myself.
You can see some of the frame construction from these pics take during its disassembly a couple years ago. The main base was traditional 2x4 and 1/2 OSB but I realized if the rest were built like that it would get real heavy, real tall, REAL QUICK! So I came up with this construction technique and for some reason it worked out well for me, your results may vary!