Earlier today a 49-year-old woman from Queens died after being pushed in front of a moving subway at the Times Square subway station in Manhattan. 
According to NYMag, a 30-year-old woman was arrested after being pointed to the police out by witnesses very quickly after the incident occurred. With 66 million annual riders, the "Times Square-42nd Street station is the busiest on the subway system", according to the New York Times, so it is no surprise that people were able to report the woman who did this.
Talking to CBS New York, NYPD Chief of Manhattan South Detectives William Aubry said that there is nothing more that the witnesses could have done to help. “The people that were on that platform did exactly what we wanted them to. If they saw something, they said something, and they saw this woman push another woman, and they flagged down two police officers that were standing on the platform.”
Police officers also told CBS that they believe that the incident was unprovoked, although sources claim that they saw the two women arguing before hand. As of late this evening, the suspect, who is confirmed to have a history of psychiatric issues, was still being questioned by police about the incident.
Person of Interest is IN custody in relation to ongoing active crime scene at Times Sq 42 st station
— Chief Joseph Fox (@NYPDTransit) November 7, 2016
Our thoughts are with the friends and family members of the victim.