I realize this is an older thread but I do have information about obtaining valuation maps, though not for Texas. For the past several years i have been maintaining a database of northeastern railroad data, a great deal of which is land valuation mapping. As more and more people come forward with material they have on hand, the larger the database has become. This in turn I offer as CD collections (also direct Dropbox downloads) for very modest pricing, just enough to maintain the service. Check it out here: https://www.nmro.org/products.htm
Cotton Belt track maps from the 1920s and 30s are a part of the J W Barriger Collection at the Barriger Railroad Library, University of Missouri, St. Louis. They should show quite a bit of detail you are looking for. Barriger was an officer of the Railroad Division of the Reconstruction Finance Corporation (RFC). Barriger was instrumental in getting Cotton Belt an RFC loan in the early 1930s.
Check your inbox, I sent you a message. It is possible that someone has already done the research you seek. I was out by the old Cotton Belt depot in Ireland, Texas back in late 2006 and my new friend pulls out an ICC valuation map for Ireland. . .
They're all sitting in College Park, MD.
In the last ten years, Railroad Model Craftsman ran a multi-part article about the process. Your first hurdle is going to be cost. The maps are actually quite large, a little over 2x4 feet, but cover small areas. A lot of them are 1 foot = quarter mile scale, so you're going to be looking at a substantial number of maps. They run $50+ a sheet, so a ten mile run is going to be a lot of pennies! As far as I know, there are no digitized copies and I'd wager the fee for getting a digitized copy isn't going to be much cheaper than a printed one (if they even do that).
Now, if you're fine with that, everything is in record group 134 and after that you need the seven digit identifier. That's easy enough to get on archives.catalog.gov. There's some tricky stuff in there, like St Louis Southwestern is 1558932, but the St Louis Southwestern of Texas is 1558933.
And this is the hardest part: there's 180 maps for the Cotton Belt in Texas and you have no way to figure out what maps you need. This is where those RMC articles come in because they explained the process of working with an archivist to pull them. Some of the info you'll need, you know.
https://www.archives.gov/research/transportation/railroad-valuation.html
There's an email there, so that's where I'd start.
I am working on a track plan for the Cotton Belt from Waco to Gatesville Texas in the early 1900's I have some Sanborn Maps for Waco, McGregor and Gatesville.
How can I find copies of the ICC valuation maps and USRA maps from this era?
Are there other maps to consider besides the maps mentioned above and the US geological maps?
Thanks,