http://images.google.com search for destroyed bridge I would find reference for the particular kind of bridge that you'd like to make... and then build it... after you have built the bridge then destroy it... that's how i roll anyways.
For your model, I'd like to see debris! I want chunks of rock/cement lying on the ground. That is, IF the bridge exploded. Judging from the bent railroad tracks and railings, that's my best guess. However, depending on how the bridge blew out or disappeared, there might not even BE rocks and stuff lying around on top of…
a good start, but here's the things that caught my eye as needing work: - the twisted railroad tracks look too thick - the flat deck of the bridge looks like a clean break, it should be a lot more jagged (maybe have chunks of concrete attached to exposed bent rebar hanging down or something) - there's no clear indication…