A good place to start is the HathiTrust Digital Library:
https://www.hathitrust.org/You can search railroad trade journals such as "Railway age", "Railroad gazette", "Railway review", "The Locomotive World", etc. Under each you can open the "Catalog record"; there you will find a list of all of the scans that are available to read. Here's the list for a search of "Railway age", for example:
https://catalog.hathitrust.org/Record/0 ... 0Age&ft=ftScroll down, click on "Full view" on a volume and you will get a compilation of three or six months worth of issues.
You can download pages as pdf or jpeg formats if you want to save stuff. Remember, most of these were printed/published a long time ago, so graphic quality is not the best, and sometimes the student volunteer making the scan didn't do a great job centering the document.
There is an absolute crap-ton of stuff in the HathiTrust Digital Library. The search engine, however, is qwerky, so search hits can be inconsistent. Also, viewing can be, but won't necessarily be limited by public domain restrictions. There's a ton of stuff here that is not yet public domain, but appears here for reasons I don't necessarily understand. Many U. S. Government reports and publications, as well as materials produced at the behest of the government, are found here as they bear no public domain restrictions.
Here's a few examples of cool railroading things I have found here over the years:
Pocket Guide to American Locomotives by Walter A. Lucas
https://babel.hathitrust.org/cgi/pt?id= ... =1up&seq=7100 Years of Steam Locomotives by Walter A. Lucas
https://babel.hathitrust.org/cgi/pt?id= ... =1up&seq=71950-52 Locomotive Cyclopedia of American Practice
https://babel.hathitrust.org/cgi/pt?id= ... =1up&seq=7Diesel-Electric Locomotive - 1946 Edition
https://babel.hathitrust.org/cgi/pt?id= ... =1up&seq=7War History of the American Railroads by Walter D. Hines
https://babel.hathitrust.org/cgi/pt?id= ... =1up&seq=9Another great repository is JSTOR, found here:
https://www.jstor.org/There's a ton of stuff here as well, for example most, if not all, of
The Railway and Locomotive Historical Society Bulletins. You have to sign up for an account to get access, but up to 100 articles and/or images are viewable monthly at no charge. I've been a free member for nine years. The JSTOR search engine is much more powerful and consistent as compared to that of HathiTrust, but both are great.
Have at it...let me know if you run into any problems.