Thermite melts things in a very noticeable way. If you mean the white shine at the bottom of the covered part that's on the right in the larger picture, I think it's just light. But look at the whole collapsed part. You can see the support beam is bent down just in front of the whitish area, and in the corresponding place on the left side of the collapsed area (lower right picture shows the bend best). Like the whole beam was pulled down by the weight in the middle, where it actually broke. It probably had been rusting and cracking for some time.

PS I have only known thermite as an amusing spectacular fireworks, but it is apparently used by railroads as a welding method. Quite interesting. http://www.railway-fasteners.com/news/what-is-railroad-thermite-welding.html So it may be true that it changes metal appearance, but if welds broke, they would be more in the middle.