Inspectors have determined the failure of a thin adhesive is to blame for bringing light rail to a five-hour halt on Inauguration Day.

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The failure of a thin adhesive is to blame for blocking light rail trips south of downtown Seattle for five hours on Jan. 20, Inauguration Day, inspections found.

That problem caused a black carbon strip to break away from a train’s rooftop pole assembly, so bare aluminum scraped an overhead wire. Some train parts became mangled and had to be sawed off.

Train trips were suspended between Stadium and Sodo stations, and at times through downtown, until 4:20 p.m. Shuttle buses moved riders through the closed area, and maintenance crews restored power just in time for the afternoon rush.

The 20-mile Link light-rail system is powered via a continuous overhead wire called the catenary. Electricity travels into the train motors through the pole assembly, called the pantograph. Springs within the pantograph press the carbon against the wire, making a continuous connection.

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Link maintenance inspectors think the carbon crumbled away, perhaps as far north as the University of Washington, while the train headed south through downtown.

Thick carbon shoes are similarly used to conduct electricity in the 1962 Seattle Center Monorail trains, which contact a power rail. Normal erosion leaves black streaks on the trackway columns.

Though this is the only such failure in light rail’s eight-year history, Sound Transit will check and refasten every train’s carbon strips during regular maintenance, without affecting passenger service, spokeswoman Kimberly Reason said.

One theory is the adhesive weakened after crews applied heating wires to pantographs at the Sodo maintenance base during recent cold weather.

“The inspections of both the pantographs and OCS [overhead catenary system] did not reveal any specific failed conditions. It is noted that the pantograph heaters were found to operate between 130 and 175 degrees F. The temperatures recorded appear to be excessive,” said a preliminary report the agency forwarded on Wednesday.

Further investigation is planned.

The light-rail system carried an average 64,000 weekday passengers as of December, almost double the year before.