What you want to do is not too difficult, but you are going to have to go with the 6 table suggestion I made back in your other thread, or else you will not be able to move the layout out of the room you build it in. The problem is that the height of the layout will be the sum of the mountains + the depth of the gorge + the thickness of the supporting benchwork. If the mountains are removeable, you will still have the depth of the gorge + the thickness of the bench work. Having the mountains removeable will leave the problem of a seam or crack along the base of both mountains. That crack can be covered by some scenery elements to minimize it, but those elements will have to be replaced if the mountains are ever moved. Your problem is that a 4' x 8' table will not go through any normal doorway without tipping it up on it's side. If you tip it on it's side to go through the door way, the width will still be at least the sum of the thickness of any roadbed, track and scenery on top of the table + the depth of the gorge and the thickness of the supporting framework. I think the minimum depth for a realistic gorge in o scale would be 10 inches. Add 4 inches for the framework and say 1 inch for subroadbed, roadbed, and track and you are at 15 inches. If you have to tip it on it's side and turn it to get it out of the room, you may end up bumping either side of the door way and doing damage to the layout. If you use the multiple sections that are no more than 2 feet wide, they can be carried out through the doorway vertically in the event of a move. This way you can also make the tunnels a permanent part of the layout.
I think your tunnels will have to be about 1/2 way along one 8 foot side of the tables, otherwise, you won't be able to make turns, unless one tunnel is curved. If you put the tunnels in the middle of one side of the layout, I would position a tunnel so that the train comes out of one tunnel about 6 inches from the end of the table. The train should go into the other tunnel about 6 inches from the end of the adjacent table. To make your gorge, cut a piece of 1" x 4" lumber the correct length to fasten to the inside of your frame and go down at the correct angle for the side of the gorge. At the bottom of the angle brace you just made fasten a new piece of 1 x 4 lumber to make a new end of the benchwork. Now go back to the top and cut off the end of the table and frame at an angle parrallel to the angle frame you just added. If you have 2 legs supporting the table in the area of the table you are cutting off, either move them bac a few inches, or shorten them to mount under the gorge. You will want to cut the frame, but leave a bit of the table top hanging over the edge of the gorge, because you want the cut the lip of the gorge with a jig saw so that it isn't perfectly straight across the rim. Nature doesn't generally do things in straight lines. You might also use your jig saw to cut out a wavy piece to go horizontally accross between the front and back of the bench work just slightly above and back from the straight pice that is the new table end. Fasten screen wire from the hardware store to the rim, the board you just cut as a profile board and the inside of the end of the table. Use plaster, paper towels or whatever method you choose to make a hardsell over the screen wire to make 1/2 of the gorge. Do a mirror image of this on the next bench to make the other side of the gorge. When you bolt these two benches together, you have gorge running between them. Your river at the bottom of the gorge can be made of cardstock, luan, or thin plywood, cut the sides to conform to the sides of the gorge, and give it a wedge cross section to fit into the bottomof the gorge. bring it all the way to the edge of your bench work and install a masonite facia board to hide the seams between the two tables. You probably should do a backdrop behind the tables, have your 2 mountains "grow" out of the backdrop and paint in the river on the backdrop winding down to join the modeled river in the gorge.
