Either mechanical failure OR terrorism.
NBC 4 (NY)
Preliminary reports suggest the 8:45 a.m. crash – involving train No. 1614 on the Pascack Valley Line from Spring Valley – was accidental or caused by operator error, according to five law enforcement officials. They stressed it was early in the investigation, though.
Too early to tell anything but it was operator error… Sure. Anything is possible.
Our governments (like NJ) run 1960s train technology on 1920s tracks. They knew they should modernize speed and breaking controls but decided to put it off until 2018.
Then again, how hard would it be for some “refugee” import to disable the breaks or hack into the controls? Of course, they could just get jobs as engineers (operators, whatever).