Certainly a lot more structure than is needed for a pipe! I guess there was some type of approach trestle or span to bring the track to the top. It looks odd sitting there with nothing leading up to it.
The tracks once ran over the top of this one. The trusses are very heavily built, with 4 eyebars per diagonal. This was typical pin-connected railroad bridge-building methodology back when this one was built.
Looking at this one, perhaps it's a structure built to hold the pipe. I can't picture any way this could carry a train. Am I missing something?