The reports are stored alphabetically in bundles. The bundle index is available here. NARA does not have Final Engineering Reports for every railroad. Final Report volumes are usually post bound sheets of paper (Form 561) with a table of contents listing page numbers for each valuation section and account # reported on.
For example, Bundle 96 Wh-Wi might have included the Wheeling & Lake Erie. The pull slip gets the set of reports and then you can see which railroads are actually there.
In this example, the W&LE was missing, but the Wheeling Terminal Railway that the W&LE had trackage rights over was included.
Although you can get permission to unbind the volumes, I found a hand held iPhone X camera provided readable images very quickly -- useful if you have limited time at NARA. Photoshop's perspective crop tool can be used to for limited straightening and the camera's 72 dpi image size can be updated to reflect the aproximate actual dpi so that Acrobat can do OCR. View Wheeling Terminal PDF.
Bundle 70 included the Pennsylvania Company Final Engineering Report Vol 6 covering the Cleveland Akron and Cincinnati Railway valuation sections in Ohio (v19 - v22). Selected pages from PCo.CA&C.v19 Hudson to Akron Junction are available here.
In addition to the Final Engineering Reports, NARA has notebooks with field notes for some of the accounts. Engineering notes for the Building account often included photos, floor plans of individual structures and handwritten statistics used in the final reports.See separate finding aid.
These reports were updated yearly on Form 588-R and occaisionally contain information on structure changes. 1927 example from Lehigh Valley