Great base! My only suggestions would be to consider adding some more diversity and density to the landscaping.
Rock outcroppings would be a logical addition -- generally, if the hill/mountain was just soil rather than solid stone, the railroad would have just done a cut, rather than blast a tunnel! There's a whole lot of easy (and fun!) methods for creating and coloring rock walls or outcroppings, ranging from commercial molds, to homemade molds, to using real rocks, even to using crumpled aluminum foil and partially-hardened plaster of paris to conform to the base. I only had room for one tunnel on my layout (and had to squeeze it in later after already laying track!), but I've used all of the above techniques (and more!) to tweak it further:
If you'd prefer to stick with the current surface for now, adding more trees, and more types of trees, as well as various sizes and types of bushes, underbrush and overgrowth, would make the scene appear a bit less manicured. Or, you could go in the other direction, adding a hiking trail (with or without hikers!) or other sign of human use to provide some additional visual interest to the scene.
BTW, none of the above is intended in any way as criticism of your progress so far -- in fact, IME most projects tend to evolve after you've had a chance to sit back after completing "phase one" and contemplate what might make the result even better! In any event, congrat's and good luck! 
