The NYC Subway system is amazing. We have the most stations of any system in the world. Other systems cover more distance (NYC is very compact) and have more riders (China, Tokyo). And a few are older (London).
But none of them are 24/7.
That is a huge deal both in terms of the fact that you can always get home and in terms of maintenance. The NYC Subway system has no maintenance window. You shut something down, you piss someone off.
So this video is fascinating in terms of the challenges of modernizing a system that you can never shut down.