At one side of Geologically Improbable Mountain, two tunnels emerge. These aren't really tunnels, just portals. The upper one is only 2" deep and the lower one is about 12" deep although they both go about 10 feet behind the mountain.
Real tunnels are maintenance problem. Animals like to live in them. Track maintenance is complicated by the inaccessibility of the track inside. Train wrecks inside are hard to remove. A common solution is to create access ports above longer tunnels. My solution was to "daylight" the region behind the portals to allow easy and level access from the other side. This method allows one to actually walk into the "tunnel" so that maintenance of the track is no more difficult than any other track.