Not a bridge expert, or really any kind of expert, really. But railroad rails are laid with a little gap to account for thermal expansion of the rail on hot days. If the expansion is more than designed for, you get buckling like this. This bridge was probably also designed to account for thermal expansion to a certain degree. It seems like more and more of our infrastructure is starting to fail, encountering heat levels it was never expected to encounter. I wonder if failures like this and worse are going to become a common headline