Two I've found interesting...
Tokyo: https://minitokyo3d.com/
India (not a subway, but the scale here makes it fascinating): https://railradar.railyatri.in/
I haven't found one for London (my hometown), but there is this interesting animation project that uses historical data to demonstrate passenger flow on the system. http://wgallia.com/#!underground
There is https://tracker.geops.de/, which integrates most of them (though not always with realtime data).
Re:NYC’s new digital subway map
by ElijahLynn on Wednesday October 21, 2020 @07:36PM
(#24853477)
New York City: https://map.mta.info/ is the new live map being discussed.
New York City: https://map.mta.info/
Washington DC: https://gis.wmata.com/metrotrain/index.html
Chicago: https://www.transitchicago.com/traintrackermap/
Boston: https://www.mbta.com/schedules/Blue/line
I'd like to see more, but that's about all I could find from cities themselves.