So... looks like there's a glimpse of one of Godzilla's non-human opponents in there. According to those who saw this year's Comic-Con feature (not the leaked teaser on last page), it's an insect/arachnid like thing with pale skin, a thin head and huge black eyes. Something that fits that description appears to be biting/tearing a train (and a bridge with the railway) apart there.

I made a GIF, with 40% of the shot's original speed.



(BTW, is it cool to post animations like this? Here's a link in case it shall be deleted: http://i39.tinypic.com/2dcbn05.gif)