A couple of things to consider.
To Carl: Saying you would absolutely jump in is a bad idea. There is a VERY short list of people I would use deadly force to defend. I've even left some family members off this list. (Due to a long history of irresponsible behavior on their part.) The guy who taught my CCW class told a story (I have absolutely no idea how true it is, but it does make one think,) about a new CCW holder who was on the street one night, and he saw a couple leaving a bar in a fight. the man pushed the woman into an alley as she struggled against him. He moved to the front of the alley to see the man with his pants down on top of the woman struggling. He pulled out his pistol and removed the top of the bad guy's head. When he went limp on top of her, the guy went to help her and as she was getting up, she yelled at him; "You idiot!! We were playing a sex game!! Why did you shoot him?!"
The reality is, many victims of sexual assault will (for the same reason they often don't report the crime,) deny that they were being assaulted and leave YOU on the hook to explain why you had to kill someone. If you yell at them to stop, and that you are talking to the police, while being ready to protect YOURSELF, you will both help the victim and cover your butt. Any action you take beyond that is off the reservation and you're on your own.
The MTA guys probably followed procedure to the letter. We can be upset that the metro cops' response time was so bad, but remember the first general order? "I will guard everything within my post and leave my post ONLY when properly relieved."? The operator of that train has a first responsibility to the passengers of that train. His jumping off the train to stop something he sees OUTSIDE of that train would be just like me leaving my kids in a car idling on the side of the road to go chase a bank robber on foot. The fact that BOTH employees did the exact same thing shows a good understanding of and compliance with MTA policies on the part of the employees. My only experience with the city metro cops is watching "Money Train", where they make it look like the whole system is crawling with cops. The reality is that it's like any other law enforcement setup. They can't be everywhere, they are not liable, you are on your own to protect yourself, and New York won't let you.