Their explanation of them is:
Under the Valuation Act of 1913, the Interstate Commerce Commission (ICC) used Railroad Valuation maps to help evaluate railroad property. The original maps were created between 1915 and 1920 by ICC and railroad employees who were tasked with inventorying most of the rail lines in the United States. The information within these maps was to be used to determine rates for transportation of freight via those rail lines.
This link will get you a search of Michigan as to what is currently online.
NATIONAL ARCHIVES CATALOG NATIONAL ARCHIVES CATALOG
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