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The 75-year-old Tower Bridge connecting Sacramento and West Sacramento may be one of the area’s icons, but the more industrial steel-truss I Street Bridge is older – built 100 years ago.

“The I Street Bridge is a very important part of our business today,” said Aaron Hunt, spokesman for the Union Pacific railroad, which owns the bridge. “We run trains across it every day of the year.”

The I Street Bridge is 363 feet long and weighs about 7 million pounds, according to a fact sheet distributed by Union Pacific.

When it was built, the swinging center of the bridge was the heaviest of its type in the world, and though it no longer holds that distinction, it remains the heaviest in the United States, according to the Sacramento Old City Association.

The bridge swings about 90 degrees on a central pedestal that is 42 feet in diameter and 84 feet high. Opening it allows boat traffic on the river, and Hunt said a Union Pacific staffer stays on-site every day to operate it. The swinging operation takes about two and a half minutes, he added.

When the bridge was renovated in 1993, some major components were replaced, including the disc upon which the bridge rotates. The controlling mechanisms were also changed over from direct-current electrical systems to hydraulic ones.

The first bridge on the site was a wagon bridge built in 1858, which was replaced in 1869 by the area’s first railroad bridge, built by the California Pacific Railroad. It, too, allowed for wagon traffic.

Construction on the current span began in 1910 by the American Bridge Company, and it was added to the National Register of Historic Places in 1982, according to Hunt.

Union Pacific took ownership of the bridge when it merged with Southern Pacific in 1996, Hunt said, and the cities of Sacramento and West Sacramento maintain the highway portion of the bridge.

Though dated, the bridge still pulls its weight, handling about 80 trains per day in addition to vehicular traffic on its upper deck.

Brandon Darnell is a staff reporter for The Sacramento Press. Follow him on Twitter @Brandon_Darnell.