I have a pretty good guess. My wife and I were at a diner in the improbable town of East Newark, NJ (called the Tops diner, place has unreal food, it is routinely ranked as the best diner in the US by some sources and all I can say is, it was really, really good food). Anyway, the diner sits by the Passaic river (real scenic, there is a concrete plant across the street from it!). Within site of the restaurant there is an old railroad drawbridge, called the NX (I looked it up), that basically was abandoned in the open position in the 1970's. I believe it was part of the old Erie system and these days I think NS has control of it via the Conrail sell off agreement.
What I was wondering is given all the steel in the bridge, why was it left like that? Was it because the cost of tearing it down would be less than the value in scrap? Or could they (unlikely, I know) have the idea that some day they might need that bridge again, and they have it in some kind of mothballs so to speak? Just seems weird that they would leave something like that standing for so long, hence the question.