DON'T LOOK DOWN.  The Kinzua Viaduct is a heroic piece of railroad engineering that last saw revenue trains in 1959.  The Erie Railroad sold it for scrap, but the scrapper was so impressed by it that he sold it to Pennsylvania for a state park.  A tourist railroad ran excursion trains across it until a survey uncovered serious structural weaknesses.

The concern now is that winds whipping through the valley on the edge of the Allegheny National Forest, about 110 miles northeast of Pittsburgh, could stress the structure enough to send parts of it crashing into the gorge. How much stress it would take to break the bridge is anyone's guess, say engineers with the state Department of Conservation and Natural Resources, which owns the bridge.

A tourist train that used to chug across the bridge was removed in June; in August, pedestrians were banned. Officials don't even want anyone under the bridge in the gorge.

"It could be very unstable with high winds, say 80 to 90 mph. Everyone wants to say, 'Well, what happens at 50 [mph]?' You can't analyze all points of the structure," said James Eppley, an engineer with the state agency. "You could lose a girder or lose a tower and it would still stand up, but we don't like those kinds of chances."

The next summer, a tornado indeed did go through the valley and break the bridge.  There is a new skydeck about to open on the remaining parts of the span.

The new deck is being built on the six surviving towers of the 2,100-foot bridge. The $4.5 million project was originally slated for completion in 2009, but it experienced delays along the way. A second phase, which will include a visitor’s center, remains in the planning stage.

This bridge is not far from the birthplace of the U.S. oil business.