This expandable railroad tunnel is based off of LEGO Lone Ranger set 79110, "Silver Mine Shootout".
LDD file included!
About this creation
This tunnel uses a ton of rock-like parts from the set, and even a couple reddish brown girder / support parts. I removed the cabin, crane, and all the features except for the waterfall. (which is missing from LDD, and thus is not in these pictures.) See that part at Bricklink for a picture: www.bricklink.com/catalogItem.asp?P=14309pb01
Rear view. The tunnel can be duplicated and connected together to form a mountain. (a very see-through mountain, but that could be a big draw at train / LEGO shows.
This tunnel can let tall objects through, such as the official caboose (10014) and my own MOC caboose, which happens to be taller the of the two. The tunnel also allows Anthony Sava's 2-8-4 Berkshire along with any of the other 8 or 7 wide engines at my disposal.