I'm going to agree with the general sentiment here that police should be deployed at transit stations as a preventative measure. However, I do have some concerns. This isn't going to solve the problem of crime and disorder in general, it's just going to displace it. My guess is, if we did this and nothing else, we would see transit be relieved, but places like public parks will see an increase in this type of activity. I guess my key issue here is this addresses the symptom and not the issue itself, and may lead to other issues in other places.
As a side note: I got off at Central Station the other day and there were about 4 or 5 police officers and a peace officer on the platform. I wasn't exactly sure what they were doing, initially I thought they may have been arresting somebody, but I didn't see a suspect, or any sort of commotion happening. It didn't seem like they were looking for anyone either, they were just standing there. I concluded they were there patrolling, but I don't know for sure.