Having worked with the fare payment hardware in an internship 5 years ago (when the current equipment was brand new), I can tell you that all of the gates have failsafes for this exact reason. They could easily put the full height vertical turnstiles into all stations, whenever there’s any emergency they all relax and allow free flow through. In the event of a power outage, or a fire alarm activation, this mode is triggered. And anyone can pull the fire alarm should a violent situation occur