Traffic Lab is a Seattle Times project that digs into the region’s transportation issues to explore the policies and politics that determine how we get around and how billions of dollars in public money are spent.

All it took was one flawed train part to block hordes of football fans trying to get home after last weekend’s Apple Cup.

Sound Transit said Thursday that a misaligned pantograph — the steel arm that draws power from high-voltage overhead wire — caused a voltage spike while the northbound light rail train was between U District and Roosevelt stations.

A full investigation of the “root cause” is underway, said the statement released by spokesperson Henry Bendon.

The mishap occurred at 5:50 p.m., and eventually a rescue train retrieved passengers from the stopped train at around 7 p.m., as transit security and fare ambassadors encouraged people to find Metro bus routes in the area. Full service didn’t resume until 8 p.m.

At least one other train was stopped behind the stall, including one at UW Station where riders saw a quick flash. After 20 minutes or so, that train was sent back to the Eastside because it couldn’t proceed toward Roosevelt and Lynnwood.

During portions of the response, trains alternated on a single track around the blocked area, running slower than normal, rider alerts said.

CEO Dow Constantine was questioned about the response Wednesday evening during an online meeting of the agency’s Citizen Oversight Panel. The panel’s chair, Charlotte Murry of Snohomish County, said she was aboard a stopped train, despite having waited an hour after the football game in hopes crowds were thinning out. She used the word “unprofessional” to describe what seemed like a disorganized response.

Constantine told the panel he called for a formal investigation by senior customer-experience staff into how the response, communication to passengers in the tunnel, and public information were handled, and how that can improve.

The pantograph flaw was located by Sound Transit’s maintenance team and has been repaired, and that railcar is back in service, according to Bendon.

The problem appears entirely different from the 2021 Apple Cup stall, when the weight of a packed train caused it to squat lower than usual, and a power cable on the train’s underside was sliced by protruding bolts on the tunnel floor, near UW Station. Those were identified by front-line workers earlier in the year but didn’t get trimmed down, a Sound Transit investigation revealed.

The most recent overhead-power incident happened July 9, when a rooftop hatch popped up and scraped an overhead power wire. Five stations in North Seattle were blocked for about nine hours.

Late the night of Aug. 1, a low-speed derailment at International District/Chinatown Station, without passengers aboard, closed downtown stations for several hours the following day, a Sunday. Workers found two bolts that sheared in a small curve there, but a detailed forensic report hasn’t been issued yet.