I started with two conventional levels, years ago. With the advent of remote and speed control, I simply had to have some of these great trains. As JR says, you often have to go up. With the addition of two trolley lines, I now have 6 levels, and can run 7 engine consists, in addition to the two trolleys.
I suggest wooden supports. These are basswood and balsa, for finer details. The table base is 1/2 inch plywood, the framework is mostly 2x4's. All is securely screwed to the frames, and begins 40 inches off the floor.
In the next two pics, you can see a small platform area that holds several structures, supported underneath by pieces of 2 X 4. Large dowels needed to be added for long runs on 3/4 inch pine planks for the highest levels.
No way I was going to re-do the first two levels, going through three rooms. If you don't have room for traditional supports, you have to improvise. Also, it would serve little purpose to connect the 60 inch highest levels with the 40 inch lowest levels. When they see the all the trains running at the same time, no one cares. I run two main DCS lines, two main conventional lines, three LCP engines on the two highest lines, and the two trolley lines.
I also built a completely separate overhead line that runs around the rec room. Building up, definitely works for me.
Hope these give you some ideas...
Best wishes,
Jerry
