Others can probably give you a more detailed explanation, but it appeared to happen at the intersection where the wire crossed the one for the other line, and the trolley apparently momentarily lost contact with the wire (maybe a slight bounce). When you have an inductive load such as a motor and the current is interrupted, it wants to keep flowing and builds up voltage causing an arc which jumps the gap.
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"A stranger's just a friend you ain't met yet." --- Dave Gardner
While hanging out in downtown Portland, I was able to shoot some footage of a MAX Train rolling through an intersection.
As the train rolled through the intersection the train appeared to generate a spark on the power wires above.
Does anybody know what causes this? https://www.youtube.com/watch?v=7U65NamUMeY
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