have you ever seen the tube system in London? it's a fully blocking barrier with a set of sliding doors that lines up precisely with the doors of the train cars when they pull in to the station.
that's probably the ideal solution, but not practical for New York because the system was never designed with precision stopping points on the platform in mind. there's a margin of error that is at least a meter or so in either direction right now.
Only on new lines with straight platforms. Older ones are almost all on curves, a guy in the sliding doors business told me they looked into but the cost was too high as each station is on a different circumference bend so they would all be custom jobs.
Only on the newest part of the newest line, the Jubilee Line Extension. (1999? something like that.)
There are no "platform edge curtain" doors on other lines, even though many other lines also have computer controlled trains. (The "driver" is still there to press a button to start the train, but many lines almost drive themselves.) Although, even on the fully manual lines, the driver is expected to stop the train precisely -- the platforms aren't long enough for inaccuracy.
that's probably the ideal solution, but not practical for New York because the system was never designed with precision stopping points on the platform in mind. there's a margin of error that is at least a meter or so in either direction right now.