The tunnel, listed on the National Register of Historic Places, was part of the original Loop that wound its way through the steep terrain of the Kenai Mountains. Designed by Frank Bartlett, an engineer for the Alaska Northern Railway, construction on the tunnel began January 16, 1906. Most of the work was undertaken during the winter months. Since snow slides blocked the south side where the rails ended, construction commenced from the north side toward the tracks. The first 250 feet of tunnel were cut with steam power and drills. At that point, the heat of the steam from the drills made the tunnel too hot for men to work, so pneumatic power (compressed air) was used to complete the project. Actual working days numbered 234; progress was made at a rate of approximately three feet per day. The tunnel was completed on October 8, 1906.