Network length per city — 215 cities equipped, 17 under construction
Metro systems worldwide — one point per CITY, not per system (12 cities operate several — Tokyo five, New York three, London two — a point per system would stack circles on identical coordinates). Radius encodes network length, the only one of three possible criteria that is both complete (215/215 cities), stable over time, and comparable across operators — station count and ridership stay readable in the tooltip.
Wikipedia, "List of metro systems", sourced system by system, accessed 2026-08-01.
Filled circles = operating systems, sized by length. Fixed-size hollow circles = systems under construction, outside the ranking: they have no length in service, giving them a proportional radius would invent a value.
Beijing (909 km, 409 stations) and Shanghai (890 km) lead a ranking dominated by Chinese metropolises, which hold seven of the world's top ten spots — the pace at which these networks are built, often several dozen kilometres a year, has no equivalent anywhere else.