Mine Tunnel

[Mohawk20]

Dune, how would you suggest to fix the foreground?
If I apply any displacement to the ground (planet 01) it also applies to the displaceble cubes, even tough they are not connected in any way...

[Dune]

That is not possible, unless they ARE connected. You could simply add another surface layer after the last to the planet and add some no color stones or fractals (not to big), and line that into the planet, but only the last surface shader before the added one to the cube, if you get what I mean.
If I have a cube and a planet with kind of the same shaders, but I don't want the main terrain to work on the cube, I simply add all detailed shaders as a child to a surface shader, so I can line that child series directly into the cube, and the whole line into the planet.

[Mohawk20]

Never mind, I was blind. In the preview the displacement of the walls seemed to be close to the camera, but in reality the floor under it lowered, revealing more wall.

Anyway, disabled glow in atmosphere for the light and placed it so it looks like it's behind the wall. And played with the foreground.
Is this better?

[Dune]

Oh yes. But I'd forget about the flower and add a good deal of desert grass patches. And reduce the size of the rail total a bit to 2/3, make the stones under the rails dirtier (added blackgrey sooty), maybe add more small scale variety to the rails' rust themselves and give the top bit a steel reflection, perhaps masked by a patchy fractal.
I love the depth in the tunnel now.

[TheBadger]

The tunnel is starting to look good. My main problem right now is the orange. Just not enough color variation making it look like a cartoon. So any problems with the tunnel structure are amplified as unbelievable by the lack of real color variation and stone variation.

As long as the color and light are real, you can get away with a lot. You can make the obviously unreal look real, just look at matte painting of fantasy scenes. IF you are stuck on the structure right now, work on the other things that TG can easily do and do well. I bet you will get it looking much like you set out for.