Hmmm...Heat-Pump Pass, well that's a natural obstruction

which is conveniently positioned to obscure the laundry area of the basement for layout viewers, as well as the big sheet-metal indoor unit for the first floor's HVAC. If you can look through the darkness and past all of the crap everywhere, you can see the clothes-rack and the indoor unit (upper left), along with a couple of paneled columns built by the previous owner:

The two track mainline tunnel at Heat-Pump Pass houses O-31 and O-42 curves, a double-track portal at one end and a two-single-track portal at the opposite end. It's the primary reason we can't run much "big stuff", as the tunnel interior is a bit tight. It's usually not a problem as most of the prewar stuff I have is relatively short, 10-inch cars probably being about the longest. Some of the postwar stuff can be a challenge, and anything scale is pretty much not happening. I installed tunnel liners made from brick-paper printed on cardstock (big pet-peeve is a tunnel that opens into a giant cavern). I like the look, but it's a big part of the reason I can't run big stuff through there.
The portals are simple, white-pine scraps found in the shop, a few bits of trim added, than shot with some textured spray-paint left over from another job. The western portal has a keystone embellishment "LL" (Lionel Lines) that the wife made for me using some foam letters she had in her old scrapbooking stash:

When we built this thing, we started by screwing the portals to the table, than built the ridge/hill behind it. The structure is all foam-board, the top being some foam partitions over a flat horizontal foam bulkhead, then paper-towel strips dipped in HydroCal.
I tried a bit of foam-carving of rocks, a mixed bag at best. I'm going to do more of that eventually, seeing if I can make something that looks even close to what you guys come up with. I need some foliage up on the top...get to that eventually.
I'm slowly getting the mess in the basement cleaned up/organized. I picked up a couple of rolling racks from Bezos, and have been making storage boxes from stuff I get over at Shipping Station. The big thing is, I'm really trying to stop acquiring stuff and work on/with what I have. There's a lifetime worth of projects laying around...slowly starting to get to them.