[CPLX]

There are many reasons not to have zones but people often miss the very serious transition/logistical problem that would be required for any change-over -- a zone system literally doubles the number of times a passenger has to swipe or scan their payment method.

That means literally millions of swipes a day that didn't exist before, each time someone has to exit the system. Many of those will necessarily fail as people don't have sufficient funds to leave the station they are in. This is a failure mode the NYC subway simply doesn't have now, all people in the system are free to leave any any time through any available low-tech door.

The idea of several hundred thousand people trying to get out through (presumably newly installed) turnstiles at grand central during rush hour while random people in front of me stall out and get trapped because their cards have insufficient funds doesn't sound fun at all. How about a tour group getting off in a very constricted station in lower Manhattan and having 90 people trapped unable to leave the platform and no room for passengers to enter? What about stations where you can't reverse direction without leaving the station, how do you head to a station you can afford to exit?

Sure in the long run anything is possible, but I don't think people who advocate for a zone system in NYC have usually thought about the actual human scale logistics required for that to work. The NYC subway is a very big complicated and crowded system and that's a major change to how passengers flow through it.