Metro systems of the world are diverse, but some of them are obviously, or not so obviously, related.
Some use the same or similar trains, have similar station design, or both — or something else is related, in a technical sense. The following list includes suggestions from a discussion at Skyscrapercity. This page last modified 4 Dec 2016. Database entries can be more recent.
German S-Bahn family Members: Frankfurt, Munich, Stuttgart, Characteristics: Using the same commuter train types (ET420, ET423). Similar underground station design. Derivatives: S-Bahn and regional train types in other German regions.
German-Austrian family Members: Berlin, Munich, Nuremberg, Vienna, Characteristics: Using similar trains. Derivatives: Hamburg Consult family.
Hamburg Consult family Members: Amsterdam, Athens, Dublin (DART), Hamburg (types DT2, DT3), Characteristics: Hamburg's metro operator had been consultant to other cities until the 1970s, so the older train types in these cities look similar.
Istanbul-Caracas family Members: Caracas, Istanbul, Characteristics: Using similar trains.
Michelin-Alstom rubber-tyred family Members: Lausanne (line M2), Lyon (lines A, B, D), Marseille, Mexico City, Montreal, Paris (lines 1, 4, 6, 11, 14), Santiago (lines 1, 2, 5), Characteristics: Rubber-tyred metro, developed by Michelin in the 1930s. Derivatives: VAL family.
US Bombardier ART family Members: Beijing (Airport Line), Detroit (people mover), Kuala Lumpur (Kelana Jaya Line), Miami (Metromover), New York (AirTrain JFK), Toronto (Scarborough RT), Vancouver (Expo Line, Millennium Line), Yongin (EverLine), Characteristics: Similar 'automated rapid transit' trains. Vancouver and Kuala Lumpur use LIM technology.
US Budd family Members: Baltimore, Miami, Characteristics: Same trains.
US Rohr Industries family Members: San Francisco (BART), Washington, Characteristics: Similar, spacious trains with carpets.
VAL family Members: Chicago (O'Hare Airport people mover), Lille, Paris (Orlyval, CDGVal airport trains), Rennes, Taipei, Toulouse, Turin, Uijeongbu, Characteristics: Sleek, small, automated, rubber-tyred metros. Platforms are usually short and have platform screen doors.
Warsaw Pact family Members: Bucharest, Budapest, Prague, Sofia, Characteristics: The same type of trains as in the Soviet family is used (or newer developments), stations unadorned.