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Has your friend already checked the Hays T. Watkins Research Library which is a part of the Baltimore & Ohio Railroad Museum? http://www.borail.org/Hays-T-Watkins-Archives.aspx
Another possibility is the Baltimore & Ohio Railroad Historical Society https://borhs.org/archives/index.html
If it was around during the Civil War, you could check with the Hupp’s Hill Cedar Creek Museum https://www.ccbf.us/?page_id=464 and even if it wasn't they may be familiar with other local history resources.