O Scale mountain for floor layout

Nice - next time maybe add a little different colored turf to the mix, so the turf isn't quite so uniform in color.

Nice work, Diego. When I glanced at your first pics, I wondered how you'd fabricated the thin-wall shell, but basing it on a commercial tunnel made that obvious. Also, my first thought was, "Where is the portal?", but then it was clear the portal was cut out to allow for the oversize cars.

You project reminded me of a drop-on tunnel I built from scratch for my brother-in-law's seasonal layout:

The base structure was created from slices of scrap styrofoam glued together and shaped with a hot wire cutter. The portals at each end were also shaped from scrap styrofoam, and the tunnel lined with cardstock printed with gray stone block. Several areas were covered with still-flexible plaster of paris, which I allowed to set up in wrinkled aluminum foil molds and then plopped over the styrofoam. The rest of the surface I covered with what I referred to as "paper ma sh*t" (toilet paper and white glue), then painted and liberally coated with assorted colors of  ground foam and finished off with a few small trees and other items I had at hand to create an informal hiking scene:

To return to your tunnel, a few thoughts for this and future efforts:

- Some sort of suggestion of a portal at the entrance would make the tunnel more believable. You might be able to salvage and relocate the original portal or just carve your own, but IMHO the entrance looks a bit unfinished/raw without some indication of human engineering. It also provides a surface to lightly dry brush with black over the track to simulate years of soot!

- I think relocating your two outside trees a bit closer to the center would also increase realism. Most trees do not grow well on steep slopes, and would be more likely to pop up on or near the center of the relatively flat top.

- I'd give some consideration to adding some rocky outcroppings on the sides and top. I overdid it a bit on my piece, but at least a few outcroppings on top and side would help 'sell' the mountain as a significant barrier in need of human intervention!

- I'd also consider trying to line at least the first few inches of the tunnel with a simulated stone face. There are very nice 3D sheets commercially available, but cardstock can be printed with a texture file (from Clever Models or elsewhere) to inexpensively simulate an inner face, and you only need it to go back a few inches to complete the effect, using pieces of styrofoam or balls of wadded paper to space it away from the inside of the tunnel as needed.

- As already noted, varying the colors, type and thickness of ground covering used will also add to realism. I'm no artist, but eventually you can get a good result by consulting prototype examples, and emulating the efforts of others. Good luck, in any event -- you're off to a good start!