The tunnel that was built for use in the construction of Grand Coulee Dam was not used to the potential it was intended for, due to a trestle below crossing the Columbia River that was too light. Instead, it became part of a tail track for a switchback.
In the photo, you can see the grade sloping behind the building in the center back. The tunnel goes through the bluff on the right. Another view of the tunnel can be seen here.
Photo brought to you by JA Phillips III.