Valuation maps were created by all US common carrier RRs in approximately the 1915-20 time frame. This was a requirement of the Interstate Commerce Commission, and the maps and reports were used by various agencies for tax and other purposes. The maps produced are incredibly data-rich, showing deed information about what entity granted the right-of-way to the railroad, various spur and stub tracks, nearby businesses, and sometimes, abandoned trackage. The Library of Congress has original copies of all of them, and recently, I found that some are now available online. In particular, the Index Maps for most MI RR are available there, although most of the individual maps have not been scanned yet. Nonetheless, knowing what Valuation Section to look in (from the index) makes it possible to know what to look for if you ever go to the Library of Congress for research, or want to commission the scanning of a particular stretch of track. I imagine that the individual sheets that have already been scanned represent research already paid for by other individuals, and that thereafter, the scan is posted to the internet.
The website for Michigan Valuation Sections is: https://catalog.archives.gov/search-within/562366?q=michigan
This area contains 1,281 individual units. After you get there, you can type in the name of an individual RR at: "Search within this Series." You can use the default "All Records," or click on "Available to access online." Then you can click on the (highlighted in blue) "item" or "file unit" which will take you to either an index map, or a series of sheets for that Valuation Section.
Once you get to an individual map, you can download it using the download button at the lower left corner of the field containing the map.
An example of an Index Map for the Michigan Central RR can be viewed at: https://catalog.archives.gov/id/165272473
As an example of one of the individual Michigan Central sheets, the following is for Valuation Section 1-B-M/S67C, downtown Kalamazoo, MI: https://catalog.archives.gov/id/155387205