Friction on the insulated cable can cause deterioration that could spark fires, said Wiedefeld, adding that if problems were found, the shutdown could continue for longer on certain part of the metro system.
"While the risk to the public is very low, I cannot rule out a potential life-safety issue here," Wiedefeld said.
The shutdown of the second-biggest U.S. subway system from 12 a.m. EDT (0400 GMT on Wednesday) to 5 a.m. EDT (0900 GMT) on Thursday would cause transport headaches for hundreds of thousands of commuters and tourists.
Washington`s metro system carries over 700,000 passengers daily, and serves as a crucial link for commuters to Washington from neighboring Maryland and Virginia.
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