I agree with Karen. If the bridge is iron and the description talks about it being cast then the iron was melted and poured into a mold to form the desired shape. That’s called casting and the result is called cast iron.

You cast things in a mould: You pour something into a pre-set mould, and let it cool into its shape, then you remove it and use it. I assume that's how they do parts for bridges, but as a non-engineer, I have no clue. It is only the context of the sentence that tells me it is so.