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Tuesday, when one side of the tracks was taken out of service just before the evening rush period, because of a smoking a stud bolt.

Though the first update related to the arcing stud bolt came quickly after the defect was reported — at 4:48 p.m. —  it was an hour before Metro tweeted that riders should consider alternatives, suggesting the delays would be significant.

It took a while — and in the process, officials narrowed the service shutdown to the stretch of tracks between Union Station and Gallery Place — but they finally found the culprit: a stud bolt, one of the metal components that helps to hold the rails in place.

Stray current can often be attributed to standing water on the tracks, debris inside a tunnel, or dirt or oil that has accumulated on the insulators that support the third rail, though Metro spokesman Dan Stessel said it was unclear what caused the stud bolt to become electrified in this case.

See Tap bolt, Screw bolt, and Stud bolt.