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Purely based on ridership (but also the one I would probably go with anyways) it's gonna be Seattle Link Light Rail with 27 million riders in 2023. The trunk line is still undergoing massive expansion that is going to double the length of the line, to say nothing of the other new lines. It's really transformed transportation in Seattle in a way we haven't seen in the US since the new metros of the 60s and 70s.
Other megaproject ridership for reference
>Grand Central Madison: 26 million (close, but no cigar)
>Second Avenue Subway: 15 million
>Expo Line: 12 million
>Silver Line: 4 million
