OK, essentially I am envisioning an all mountain railroad built on an around-the-wall platform with no width wider than 3'. The mountain scenery would rise from the front of each piece to the "wall" at the back with the sole exception of your branch-mainline junction yard. From your branch junction yard, your mainline trains enter tunnels at each end (after departing the yard area) and stay under the mountains in storage (on the other 16' side) until their next scheduled appearance at the junction yard. Of course, the mainlines would have to be at the back of the train platforms because they will remain on the primary level, but you would be able to make your yard area at least 6-tracks wide allowing for 3 different trains (including some passenger) to run in each direction.
Your branch, on the other hand starts to rise up into the mountains (fully or partially visible as you choose) to service the coal mine(s). Here, you can wind the track back and forth between the front and back of the train platform as the trains rise to the mine locations on the mountains, and you might even have 2 or 3 different levels of mines.
There are only two issues with this idea that I can't provide suggestions on how to fix by thought. First, at your last mine (branch end), you will need a small yard and small turntable (or wye, which is easily possible in a corner) to turn the road engine. And, silly me, I just realized that you will need some way to turn and service the branch engines at the junction yard! Second, rather than run your mainline trains in pure (mostly hidden) circles, you may desire to include reverse loop storage to add more direction variety. In any event, you would be free to run large, heavy engines on your mainlines (where your passenger trains might also run) and then use smaller engines (and tighter radii curves) to reach your mines.
I hope this makes more sense. To flesh it out before you build, I would recommend acquiring either the SCARM or the AnyRail track planning programs. They are both Windows-based programs and are free to use up to a limited number of track pieces, and many members here can give you more advice once they see how you implement your ideas.
