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The tunnel goes through Maynard Mountain for 13,300 ft. (2.5 miles) and is the longest combined rail and highway tunnel in North America. The original railroad tunnel was expanded in 1998 to include vehicle traffic.
The tunnel is bored directly through the mountain rock and is extremely stable.
When the 1964 Good Friday Eathquake hit, the epicenter was very nearby in the Prince William Sound. The 9.2 quake was the largest in North American history and the tunnel suffered no significant structural damage and no cave-ins.
The design allows a single lane of traffic to drive over the railroad tracks, so cars are allowed through one direction at a time in 30 minute increments. The drive takes about 6 minutes at 25 MPH to travel through the tunnel.