This is one of the most significant bridges in southern Michigan, both for its age, its association with the state's first railroad, and for being one of only a few large-scale stone arch bridges in the state. It may be the second oldest bridge in Michigan. I am not 100% sure on that, but so far the only Michigan bridge that I have discovered to be older is the Ackley Covered Bridge, which was built in 1832 in Pennsylvania and moved to Greenfield Village in 1937. So if that is considered cheating since it was built in another state, the Adrian bridge may "just possibly" be truly the oldest in Michigan. Further research is needed to confirm this, however.
Paul,
Not counting covered bridges, this is the oldest bridge I have associated a construction date with in Michigan. Unless there are other stone rail bridges hidden away in less visible locations that we do not know of, this could indeed be the oldest.