I don't think Tokyo is number 1 either. https://en.m.wikipedia.org/wiki/List_of_metro_systems …
...but something like the S-Bahn in Germany falls into the same category (and is functionally indistinct from the U-Bahn)
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That is, if you count Berlin's "subway ridership" as just it's U-Bahn, you're missing half the picture
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