One of the challenges of working with a "shelf" type layout (mine is only 27" deep in most places) is trying to make use of the available space for some scenic features. There are few features more interesting than tunnels cutting through big mountains, but the operative word "big" can be elusive. Another challenge for me personally, not to overstate my situation, is working on scenery from a seated position. Here's the link to my layout thread on OGR, which explains more:
I decided to build a mountain across one end of the layout, but I also wanted to keep a small spur line in the foreground with an operating ice house. The tunnel would be approximately 9 feet long and curved along its entire length. One big consideration was figuring out some sort of detachable segments to provide access for track maintenance or even derailments. A related consideration was making the mountain in modules. I could then place each section on a workbench to do most of the scenery work.
Here's what I came up with--a method not unlike building a boat upside down. I cut "ribs" from 3/4-inch insulation board, using a 100-watt hot knife to cut the outlines and tunnel openings. [NOTE: Polystyrene foam board gives off toxic smoke, so either insure that you have ample ventilation in your space or wear an appropriate respirator. I have a heavy-duty exhaust fan installed in my train room for that purpose.] The spars to maintain rib spacing were made from scrap pieces of insulation board glued and screwed to the ribs. I used clear Gorilla glue and 3-inch #6 sheetrock screws, which work nicely using a small cordless screwdriver. The tunnel openings were tested frequently with the longest cars in my inventory to make sure I allowed enough clearance.
I built the skeleton in four sections. Once the sections formed a good fit, I covered them with plaster cloth. However, instead of wetting the cloth first, I cut dry strips and fastened them to the ribs with a staple gun. Once the whole section was covered, I sprayed the surface liberally with a spray bottle and activated the plaster. It drooped a little, creating a "covered wagon" effect, but that will be mostly hidden by scenery.
The middle two sections will be removable (see photos). I fastened a "rock outcropping" made from foam to the left-hand removable piece: it will serve as a sort of pull knob for detaching that segment, after which the right hand segment is easy to lift off. The two end pieces will be firmly secured to the layout. The section at far left is by far the largest. It barely fits through a 36-inch doorway, yet I can easily hold it with one hand (although it's bound to get heavier as scenery details are applied). It is topped off with a neat Lionel rotating beacon.
That's my progress so far. The next step will be to attach Shaper Sheet to disguise the joints between the segments, then begin some light plaster work and start adding scenery. I'm trying to force perspective, so the sheer side walls will have minimal vegetation. The tops will get the full treatment, and I'm making some of my own trees.
Hopefully I'll have some more progress to share soon.