Hacker News Reader

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).

New York City: https://map.mta.info/ is the new live map being discussed.

fwiw I find the others (DC, Chi, BOS) easier to view/pan/explore