A truly impressive effort, as limited as it might be. Congratulations on bringing light to the holdings of the library!
The first comment I would make is to save the file in the most easily accessible software format possible for those who may not have Excel. I would suggest staying with the XLS format rather than going to the XLSX format. The newer (and more exclusive) format most likely doesn't gain you anything, and people with older spreadsheet programs might run into an obstacle with it. Just a suggestion from someone who does not typically use M$ Excel.
While it is most likely necessary, make certain that the compression program you use to zip files continues to function correctly in the future. I have found that to be a problem with some older compressions in early versions of MS software. Just an observation.
Some of the first entries do not mention specific road names, car series, or types. This may be something that simply wasn't on the drawings or wasn't picked up when those drawings were scanned. Again, an observation.
While it may be too late for some, it might be useful if the road initials were in a separate, sortable column to facilitate locating specific cars.
And it appears you DO have something of interest to me! SS-16294.tif appears to be one of the early BA&P two-bay hoppers, although it might be one of the earlier cars, which would still be of interest.
Regarding reporting marks, I notice that some of the initials have spaces in them. the above-mentioned drawing refers to "B A & P Hopper car" with spaces between the initials. If I'm searching for a reporting mark I don't put in spaces because the railroads and the equipment registers didn't. In this case it would be more useful to use "BA&P". Note that this may simply be a typo. Personally I favor leaving in the ampersand (&), as it is correct for the era, but it's not unreasonable to do a second search if it's not there. Where road names are spelled out, such as "General drawing of 60000 lbs. cap stock car Bangor & Aroostook", it would be helpful to have reporting marks following the name for searching. This might help avoid spelling errors derailing a search.
Just some quick thought. Again, congratulations on such an effort!!