In 1887, construction on the tunnel began. Crews worked from both ends. The north end was called Portal and the south end Amazon. When the tunnel was built, it was the longest railroad tunnel in Montana at 6,145 feet. The first train rolled through the tunnel on October 24, 1888.

The large wooden doors were added at each end and kept closed in winter to keep snowdrifts out and the warm air in. These doors had to be opened every time a train passed through and needed to be closed afterward.