It derailed once before, same spot. Bad ground.
A couple of other threads if anyone is interested in the comments thereon:
http://www.freerepublic.com/focus/f-news/3289215/posts 242 replies
50 Hurt After Amtrak Train Derails in Philly ^
http://www.freerepublic.com/focus/f-news/3289254/posts 17 replies
At least five dead, 50 injured in Philadelphia Amtrak derailment ^