Personally I can't make heads or tails of what's going on in that scene.
I do know a few details though after watching the vid on my Quicktime 7 player frame by frame, and you can just barely glimpse a shopping cart sized flap of pinkish flesh in the opening where the train was opened up. It's so brief you almost don't notice it, but if it is Godzilla's head then I can see that being the skin in the corner of his mouth, though it seems a little small for Godzilla if he really is as large as the rest of the trailer makes him out to be. Another possibility is that it was one of the Muto creatures biting down on the train, as we know they will be big SOB's even if they can't compete with the Big-G himself.
About the only other thing I know after examining that scene that closely was that the train was outdoors, not in a subway. Whatever was happening to that train was happening on an outdoors track at night. Why? Because you can see building lights in the background and there are pillars holding up the track.
So, even after a critical examination I don't know what to tell you other than it was an outdoor train track and whatever was tearing it apart seemed pretty big but not big enough to be Godzilla. We won't know for sure what it is that's doing the poor train in until the movie itself comes out, but at least I hope I've provided a bit of peace of mind about the issue.
