When you are building tunnels you should make them as high as possible,

or at least higher than an inter-modal double stack. I got 1 tunnel high 

enough but not the other. It is about a 1/16th of an inch to low and hangs

the train in the second tunnel.

We used screen for support and vinyl spackle to make mountains and landscaping. It requires less support than do traditional ways of building 

and is much stronger. I used to make the same thing out of wood putty

over 20 years ago.  

The Rock Face is Limestone. When you are stacking the pieces you will

have to try several different combinations they need to interlock and still

leave enough room to allow the stones that will come above lean slightly

towards the mountain. You will want to take your stone slightly above the 

top of the cliff, it makes it easier to tie into the top. If the area between the 

stones are to large you can use much smaller stones to fill it all in. For the 

small cracks you can use shrub, grass, sand, or similar material to fill it in.

I used all of it to get it done. I will discuss how to tie the stones in over the

tunnel opening.  When installing the limestone use an all purpose white glue 

[like Elmer's glue all]