> I wonder if Tokyo decided to build a few more lines/expand the capacity of the existing ones (e.g. by building extra tunnels or elevated lines on existing rights of way), if there is a point where the train capacity really could exceed the peak demand the city could produce
I doubt it - more capacity just means more population growth and more demand. Central London has reached a point where they're worried about the pedestrian capacity on some central bridges. But in what's already the most populous and second most economically productive metro area in the world, of course there are very real benefits to growing further.
> more interestingly, I wonder how many trips don't happen in car-dependent cities (or ones with decrepit transit like NYC) because of how inefficient their infrastructure is at moving people.
NYC does pretty well, but yeah of course available transport affects what people can do. I saw an interesting conception of this as the "effective size" of a city, e.g. how many people and jobs there are within a 30- or 60-minute commute radius.
> A noticeable effect to me of all this transit is that the car traffic on the roads usually is pretty minimal.
Not my experience. Tokyo gets as gridlocked as anywhere else during rush hour, and they're constantly building new roads as well (including e.g. a fourth expressway ringroad).